Natasha is going to fly to Mars. She needs to build a rocket, which consists of several stages in some order. Each of the stages is defined by a lowercase Latin letter. This way, the rocket can be described by the string — concatenation of letters, which correspond to the stages.

There are n stages available. The rocket must contain exactly k of them. Stages in the rocket should be ordered by their weight. So, after the stage with some letter can go only stage with a letter, which is at least two positions after in the alphabet (skipping one letter in between, or even more). For example, after letter 'c' can't go letters 'a', 'b', 'c' and 'd', but can go letters 'e', 'f', ..., 'z'.

For the rocket to fly as far as possible, its weight should be minimal. The weight of the rocket is equal to the sum of the weights of its stages. The weight of the stage is the number of its letter in the alphabet. For example, the stage 'a 'weighs one ton,' b 'weighs two tons, and' z' — 26 tons.

Build the rocket with the minimal weight or determine, that it is impossible to build a rocket at all. Each stage can be used at most once.

Input

The first line of input contains two integers — n and k (1 ≤ k ≤ n ≤ 50) – the number of available stages and the number of stages to use in the rocket.

The second line contains string s, which consists of exactly n lowercase Latin letters. Each letter defines a new stage, which can be used to build the rocket. Each stage can be used at most once.

Output

Print a single integer — the minimal total weight of the rocket or -1, if it is impossible to build the rocket at all.

Examples

Input

5 3
xyabd


Output

29

Input

7 4
problem


Output

34

Input

2 2
ab


Output

-1

Input

12 1
abaabbaaabbb


Output

1

Note

In the first example, the following rockets satisfy the condition:

  * "adx" (weight is 1+4+24=29);
  * "ady" (weight is 1+4+25=30);
  * "bdx" (weight is 2+4+24=30);
  * "bdy" (weight is 2+4+25=31).



Rocket "adx" has the minimal weight, so the answer is 29.

In the second example, target rocket is "belo". Its weight is 2+5+12+15=34.

In the third example, n=k=2, so the rocket must have both stages: 'a' and 'b'. This rocket doesn't satisfy the condition, because these letters are adjacent in the alphabet. Answer is -1.
			     
			import java.io.*;
			        		    import java.math.*;
			        		    import java.util.*;
		import java.util.regex.Pattern;
		
		import javafx.scene.shape.Line;
			    	     
			        		    public class Main{
			        		  	private static InputStream stream;
			        		    	private static byte[] buf = new byte[1024];
			        		    	private static int curChar;
			        		    	private static int numChars;
			        		    	private static SpaceCharFilter filter;
			        		    	private static PrintWriter pw;
			        		    	private static long count = 0,mod=(long)1e9+7;
			        		    //	private static TreeSet<Integer> ts=new TreeSet[200000];
			public final static int INF = (int) 1E9;
			
			public static void main(String args[]) {
		    	
				InputReader(System.in);
				pw = new PrintWriter(System.out); 
			    new Thread(null ,new Runnable(){
			       public void run(){
			           try{
			        		
			               test();
		
			               pw.close();
			           } catch(Exception e){
			               e.printStackTrace();
			           }
			       }
			   },"1",1<<26).start();
			   	   }
			    static StringBuilder sb;
			    public static void test() throws IOException{
			    	 sb=new StringBuilder();
			    	
			    	 int t=1;
			    	 
			    	 while(t-->0){
			    		
			    		solve();
			    		//pw.println();
			    		//sb.append("\n");
			    	}
			    	//pw.println(sb);
			    }
			    
			    public static long pow(long n, long p,long mod) {
					if(p==0)
						return 1;
					if(p==1)
						return n%mod;
					if(p%2==0){
						long temp=pow(n, p/2,mod);
					return (temp*temp)%mod;
					}else{
						 	long temp=pow(n,p/2,mod);
						 	temp=(temp*temp)%mod;
						 	return(temp*n)%mod;
						 	
					}
				}
			    public static long pow(long n, long p) {
					if(p==0)
						return 1;
					if(p==1)
						return n;
					if(p%2==0){
						long temp=pow(n, p/2);
					return (temp*temp);
					}else{
						 	long temp=pow(n,p/2);
						 	temp=(temp*temp);
						 	return(temp*n);
						 	
					}
				}
			    public static void Merge(long a[],int p,int r){
			        if(p<r){
			            int q = (p+r)/2;
			            Merge(a,p,q);
			            Merge(a,q+1,r);
			            Merge_Array(a,p,q,r);
			        }
			    }
			    public static void Merge_Array(long a[],int p,int q,int r){
			       long b[] = new long[q-p+1];
			        long c[] = new long[r-q];
			        for(int i=0;i<b.length;i++)
			            b[i] = a[p+i];
			        for(int i=0;i<c.length;i++)
			            c[i] = a[q+i+1];
			        int i = 0,j = 0;
			        for(int k=p;k<=r;k++){
			            if(i==b.length){
			                a[k] = c[j];
			                j++;
			            }
			            else if(j==c.length){
			                a[k] = b[i];
			                i++;
			            }
			            else if(b[i]<c[j]){
			                a[k] = b[i];
			                i++;
			            }
			            else{
			                a[k] = c[j];
			                j++;
			            }
			        }
			    }
			    
			  
				public static long gcd(long x, long y) {
					if (x == 0)
						return y;
					else
						return gcd( y % x,x);
				}	
				
				public static boolean isPrime(int n) {
					if (n <= 1)
						return false;
					if (n <= 3)
						return true;
			 
					if (n % 2 == 0 || n % 3 == 0)
						return false;
			 
					for (int i = 5; i * i <= n; i = i + 6)
						if (n % i == 0 || n % (i + 2) == 0)
							return false;
			 
					return true;
				}
				
			        static LinkedList<Integer> adj[],adj2[];
			        static boolean Visited[];
			        static HashSet<Integer> exc;
			        static long oddsum[]=new long[1000001];
			        static long co=0;
			        static int indeg[],outdeg[];
	  		      
			    
			    	private static void buildgraph(int n){
			    		adj=new LinkedList[n+1];
			    		adj2=new LinkedList[n+1];
			    		Visited=new boolean[n+1];
			    		levl=new int[n+1];
			    		indeg=new int[n+1];
			    		outdeg=new int[n+1];
			    		for(int i=0;i<=n;i++){
			    			adj[i]=new LinkedList<Integer>();
			    			adj2[i]=new LinkedList<Integer>();
			    		}
			
			
			    	}
			      
			       
			        static int[] levl;
			        static int[] eat;
			        static int n,m;
			        static int price[];
			        //ind frog crab
			    
			        
			        static long d,x,y;
					static long modInverse(long A, long M)
					{
					extendedEuclid(A,M);
					return (x%M+M)%M;    //x may be negative
					}
					static void extendedEuclid(long A, long B) {
					if(B == 0) {
					d = A;
					x = 1;
					y = 0;
					}
					else {
					extendedEuclid(B, A%B);
					long temp = x;
					x = y;
					y = temp - (A/B)*y;
					}
					}
			        static double distance(int x1,int y1,int x2,int y2){
			        	return Math.sqrt((x1-x2)*(x1-x2) + (y1-y2)*(y1-y2));
			        }
			        static double area(int x1, int y1, int x2, int y2, int x3, int y3)
			        {
			           return Math.abs((x1*(y2-y3) + x2*(y3-y1)+ x3*(y1-y2))/2.0);
			        }
			        static boolean isInside(int x1, int y1, int x2, int y2, int x3, int y3, int x, int y)
			        {   
			           double A = area (x1, y1, x2, y2, x3, y3);
			         
			           double A1 = area (x, y, x2, y2, x3, y3);
			         
			           double A2 = area (x1, y1, x, y, x3, y3);
			         
			           double A3 = area (x1, y1, x2, y2, x, y);
			           
			          return (A == A1 + A2 + A3);
			        }
			      
					static int col[];
			    public static boolean isVowel(char c){
			    	if(c=='a' || c=='e'||c=='i' || c=='o' || c=='u')
			    		return true;
			    	return false;
			    }
			static long arr[], lazy[],ans[];
			static int c[],nodes[];
			static int dp[][];
			static long a[][];
			
			private static boolean isPrim(long n,int p){
				for (long i = 2; i * i <= n; i++) {
			        if (n % i == 0 && i <= p) return false;
			    }
			    return true;
			}
			//price - starting index of string with given hash
			//rating-count of number of time of a hash
			static ArrayList<String> all_perm;
			static boolean[][] visited1;
			static  boolean won;
			static boolean containsCycle;
			static ArrayList<Integer> anss;
			static int index = 0;
			static int trav[],subtree[];
		public static void solve() throws IOException{
				int n=nextInt();
				int k=nextInt();
				String s = nextLine();
				int freq[] = new int[26];
				for(int i=0;i<s.length();i++){
					freq[s.charAt(i)-'a']++;
				}
				int tot_weight=0;
				for(int i=0;i<26 && k>0;i++){
					if(freq[i]>0){
						tot_weight+=(i+1);
						k--;
						i++;
					}
				}
				if(k>0){
					pw.println(-1);
					return;
				}
				pw.println(tot_weight);
		}
		static int res=0;
		private static void dfs(int start, int par){
			Visited[start] = true;
			trav[index] = start;
			index++;
			for(int x: adj[start]){
				if(!Visited[x]){
					dfs(x, start);
				}
			}
			subtree[start]++;
			subtree[par]+=subtree[start];
		}
		private static boolean isPalindrome(int freq[]){
			int odd=0;
			for(int i=0;i<26;i++){
				if(freq[i]%2!=0){
					odd++;
				}
			}
			if(odd<=1)
				return true;
			return false;
		}
			
			private static int mySoln(int n,int a,int b){
				int tempa=Math.min(a, b);
				int tempb=Math.max(a, b);
				ArrayList<Integer> list = new ArrayList<Integer>();
				for(int i=1;i<=n;i++){
					list.add(i);
				}
				
				int round=1;
				while(true){
					ArrayList<Integer> temp = new ArrayList<Integer>();
					
					for(int i=0;i<list.size();i+=2){
						if(list.get(i)==tempa && list.get(i+1)==tempb){
							
							return round;
						}
						if(list.get(i)==tempa ||  list.get(i)==tempb){
							temp.add(list.get(i));
						}else if(list.get(i+1)==tempa || list.get(i+1)==tempb){
							temp.add(list.get(i+1));
						}else{
							temp.add(list.get(i));
						}
					}
					
					list.clear();
					for(int i:temp){
						list.add(i);
					}
				
					round++;
				
					
				}
			}
			private static int othersoln(int n,int a,int b){
			for(int i=16;i>=0;i--){
				if(BB(a,i) ^ BB(b,i)){
					int m=i+1;
					
						return m;
								
			}
			}
			return 0;
			
			}
			private static boolean BB(int n,int i){
				return (n & (1<<i)) >0;
			}
					private static boolean check(int x,int y,int n,int m){
				if(x>=0 && x<n && y>=0 && y<m)
					return true;
				return false;
			}
					
					
					
								
			
			    private static boolean Check(int x,int y,int n){
					if(x>=0 && x<n && y>=0 && y<n)
						return true;
					return false;
				}
			    
			    public static String reverseString(String s) {
					StringBuilder sb = new StringBuilder(s);
					sb.reverse();
					return (sb.toString());
				}
			    
			  /*  
			   private static void BFS(int sou){
			    	Queue<Integer> q=new LinkedList<Integer>();
			    	q.add(sou);
			    	Visited[sou]=true;
			    	levl[sou]=0;
			    	while(!q.isEmpty()){
			    		int top=q.poll();
			    		
			    		for(int i:adj[top]){
			    			//pw.println(i+" "+top);
						if(!Visited[i])
						{
							
							q.add(i);
							levl[i]=levl[top]+1;
						}
						
						Visited[i]=true;
						
						}
					}
				}*/
				
			
			
			
			    		      
			    		   /* private static void kahn(int n){
			    		    	
			    		    	PriorityQueue<Integer> q=new PriorityQueue<Integer>();
			    				for(int i=1;i<=n;i++){
			    					if(indeg[i]==0){
			    						q.add(i);
			    					}
			    				}
			    				while(!q.isEmpty()){
			    					int top=q.poll();
			    					st.push(top);
			    					for(Node i:adj[top]){
			    						indeg[i.to]--;
			    						if(indeg[i.to]==0){
			    							q.add(i.to);
			    						}
			    					}
			    				}
			    		    }
			    		        
			    		        static int state=1;
			    		        static long no_exc=0,no_vert=0;
			    		      static Stack<Integer> st;
			    		      static HashSet<Integer> inset;
			    		     /*   private static void topo(int curr){
			    		        	
			    		        	Visited[curr]=true;
			    		        	inset.add(curr);
			    		        	for(int x:adj[curr]){
			    		        		if(adj[x].contains(curr) || inset.contains(x)){
			    		        			state=0;
			    		        			return;
			    		        		}
			    		        		if(state==0)
			    		        			return;
			    		        		
			    		        	}
			    		        	st.push(curr);
			    		        	
			    		        	inset.remove(curr);
			    		        }*/
			    		        static HashSet<Integer> hs;
			    		       
			    		     
			    		     	static boolean prime[];
								static int spf[];
								public static void sieve(int n){
									prime=new boolean[n+1];
									spf=new int[n+1];
									
									Arrays.fill(spf, 1);
									Arrays.fill(prime, true);
								prime[1]=false;
									spf[2]=2;
									
								for(int i=4;i<=n;i+=2){
									spf[i]=2;
								}
								for(int i=3;i<=n;i+=2){
									if(prime[i]){
										spf[i]=i;
										for(int j=2*i;j<=n;j+=i){
											
											prime[j]=false;
										if(spf[j]==1){
											spf[j]=i;
										}
										}
									}
								}
								
									
								}
			    		     	
			    		    	// To Get Input
			// Some Buffer Methods
			 
			    		     	public static void sort(long a[]){
			    		     		Merge(a, 0, a.length-1);
			    		     	}
			    		    	public static void InputReader(InputStream stream1) {
			    		    		stream = stream1;
			    		    	}
			 
			    		    	private static boolean isWhitespace(int c) {
			    		    		return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
			    		    	}
			 
			    		    	private static boolean isEndOfLine(int c) {
			    		    		return c == '\n' || c == '\r' || c == -1;
			    		    	}
			 
			    		    	private static int read() {
			    		    		if (numChars == -1)
			    		    			throw new InputMismatchException();
			    		    		if (curChar >= numChars) {
			    		    			curChar = 0;
			    		    			try {
			    		    				numChars = stream.read(buf);
			    		    			} catch (IOException e) {
			    		    				throw new InputMismatchException();
			    		    			}
			    		    			if (numChars <= 0)
			    		    				return -1;
			    		    		}
			    		    		return buf[curChar++];
			    		    	}
			 
			    		    	private static int nextInt() {
			    		    		int c = read();
			    		    		while (isSpaceChar(c))
			    		    			c = read();
			    		    		int sgn = 1;
			    		    		if (c == '-') {
					sgn = -1;
					c = read();
				}
				int res = 0;
				do {
					if (c < '0' || c > '9')
						throw new InputMismatchException();
					res *= 10;
					res += c - '0';
			    		    			c = read();
			    		    		} while (!isSpaceChar(c));
			    		    		return res * sgn;
			    		    	}
			 
			    		    	private static long nextLong() {
			    		    		int c = read();
			    		    		while (isSpaceChar(c))
			    		    			c = read();
			    		    		int sgn = 1;
			    		    		if (c == '-') {
					sgn = -1;
					c = read();
				}
				long res = 0;
				do {
					if (c < '0' || c > '9')
						throw new InputMismatchException();
					res *= 10;
					res += c - '0';
			    		    			c = read();
			    		    		} while (!isSpaceChar(c));
			    		    		return res * sgn;
			    		    	}
			 
			    		    	private static String nextToken() {
			    		    		int c = read();
			    		    		while (isSpaceChar(c))
			    		    			c = read();
			    		    		StringBuilder res = new StringBuilder();
			    		    		do {
			    		    			res.appendCodePoint(c);
			    		    			c = read();
			    		    		} while (!isSpaceChar(c));
			    		    		return res.toString();
			    		    	}
			 
			    		    	private static String nextLine() {
			    		    		int c = read();
			    		    		while (isSpaceChar(c))
			    		    			c = read();
			    		    		StringBuilder res = new StringBuilder();
			    		    		do {
			    		    			res.appendCodePoint(c);
			    		    			c = read();
			    		    		} while (!isEndOfLine(c));
			    		    		return res.toString();
			    		    	}
			 
			    		    	private static int[] nextIntArray(int n) {
			    		    		int[] arr = new int[n];
			    		    		for (int i = 0; i < n; i++) {
			    		    			arr[i] = nextInt();
			    		    		}
			    		    		return arr;
			    		    	}
			 
			    		    	private static long[][] next2dArray(int n, int m) {
			    		    		long[][] arr = new long[n][m];
			    		    		for (int i = 0; i < n; i++) {
			    		    			for (int j = 0; j < m; j++) {
			    		    				arr[i][j] = nextLong();
			    		    			}
			    		    		}
			    		    		return arr;
			    		    	}
			    		    	private static char[][] nextCharArray(int n,int m){
			    		    		char [][]c=new char[n][m];
			    		    		for(int i=0;i<n;i++){
			    		    			String s=nextLine();
			    		    			for(int j=0;j<s.length();j++){
			    		    				c[i][j]=s.charAt(j);
			    		    			}
			    		    		}
			    		    		return c;
			    		    	}
			 
			    		    	private static long[] nextLongArray(int n) {
			    		    		long[] arr = new long[n];
			    		    		for (int i = 0; i < n; i++) {
			    		    			arr[i] = nextLong();
			    		    		}
			    		    		return arr;
			    		    	}
			 
			    		    	private static void pArray(int[] arr) {
			    		    		for (int i = 0; i < arr.length; i++) {
			    		    			pw.print(arr[i] + " ");
			    		    		}
			    		    		pw.println();
			    		    		return;
			    		    	}
			 
			    		    	private static void pArray(long[] arr) {
			    		    		for (int i = 0; i < arr.length; i++) {
			    		    			pw.print(arr[i] + " ");
			    		    		}
			    		    		pw.println();
			    		    		return;
			    		    	}
			 
			    		    	private static void pArray(boolean[] arr) {
			    		    		for (int i = 0; i < arr.length; i++) {
			    		    			pw.print(arr[i] + " ");
			    		    		}
			    		    		pw.println();
			    		    		return;
			    		    	}
			 
			    		    	private static boolean isSpaceChar(int c) {
			    		    		if (filter != null)
			    		    			return filter.isSpaceChar(c);
			    		    		return isWhitespace(c);
			    		    	}
			 
			    		    	private interface SpaceCharFilter {
			    		    		public boolean isSpaceChar(int ch);
			    		    	}
			    		    	
			
			}
			    		    
			    		        class Node{
			    		    	int to;
			    		    	long dist;
			    		    	Node(int to,long dist){
			    		    		this.to=to;
			    		    		this.dist=dist;
			    		    	}
			    		    	
			    		    }
			    		        class Qu{
			    		    		long a0,a1,a2,a3;
			    		    		public Qu(long a0,long a1,long a2,long a3) {
			    		    			this.a0=a0;
			    		    			this.a1=a1;
			    		    			this.a2=a2;
			    		    			this.a3=a3;
			    		    		}
			    		    }
			    		
			class Dsu{
			private int rank[], parent[] ,n;
			long str[],si[],larg[];
			long max=0;
			int cnt=0;
			private static int[] parent1;
			Dsu(int size){
				this.n=size+1;
				rank=new int[n];
				cnt=n-1;
				//parent=new int[n];
			    		    		    		parent=new int[n];
			    		    		    		str=new long[n];
			    		    		    		si=new long[n];
			    		    		
			    		    		    		larg=new long[n];
			    		    		    		makeSet();
			    		    		
			    		    		    	}
			    		    		    	
			    		    		    	void makeSet(){
			    		    		    		for(int i=0;i<n;i++){
			    		    		    			parent[i]=i;
			    		    		    			si[i]=1;
			    		    		    		}
			    		    		    	}
			    		    		    	
			    		    		    	int find(int x){
			    		    		    		if(parent[x]!=x){
			    		    		    			
			    		    		    			parent[x]=find(parent[x]);
			    		    		    		}
			    		    		    		return parent[x];
			    		    		    	}
			    		    		    	
			    		    		    	void join(int ind,long val){
			    		    		    		if(str[ind]==0){
			    		    		    			str[ind]=val;
			    		    		    			larg[ind]=val;
			    		    		    			max=Math.max(max, str[ind]);
			    		    		    		}
			    		    		    		int prev_ind=(ind-1);
			    		    		    		int next_ind=(ind+1);
			    		    		    		if(prev_ind>0 && larg[prev_ind]!=0){
			    		    		    			union(prev_ind,ind);
			    		    		    		}
			    		    		    		if(next_ind<n && larg[next_ind]!=0){
			    		    		    			union(next_ind,ind);
			    		    		    		}
			    		    		    		
			    		    		    	}
			    		    		    
			    		    		    	
			    		    		    	boolean union(int x,int y){
			    		    		    		int xRoot=find(x);
			    		    		    		int yRoot=find(y);
			    		    		    
			    		    		    	
			    		    		    		if(xRoot==yRoot)
			    		    		    			return false;
			    		    		    		if(rank[xRoot]<rank[yRoot]){
			    		    		    		parent[xRoot]=yRoot;
			    		    		    		
			    		    		    		larg[yRoot]=Math.max(larg[xRoot], larg[yRoot]);
			    		    		    		
			    		    		    		si[yRoot]+=si[xRoot];
			    		    		    		str[yRoot]=larg[yRoot]*si[yRoot];
			    		    		    			max=Math.max(str[yRoot], max);
			    		    		    		}else if(rank[yRoot]<rank[xRoot]){
			    		    		    			parent[yRoot]=xRoot;
			     
			    		    		    			larg[xRoot]=Math.max(larg[xRoot], larg[yRoot]);
			        		    		    		
			        		    		    		si[xRoot]+=si[yRoot];
			        		    		    		str[xRoot]=larg[xRoot]*si[xRoot];
			    			    		    		max=Math.max(str[xRoot], max);
			    		    		    		}else{
			    		    		    			parent[yRoot]=xRoot;
			    		    		    			rank[xRoot]++;
			     
			    		    		    			larg[xRoot]=Math.max(larg[xRoot], larg[yRoot]);
			        		    		    		
			        		    		    		si[xRoot]+=si[yRoot];
			        		    		    		str[xRoot]=larg[xRoot]*si[xRoot];
			    			    		    		max=Math.max(str[xRoot], max);
			    		    		    		
			    		    		    		}
			    		    		    		cnt--;
			    		    		    		return true;
			    		    		    	}
			    	
			    		    		     
			    		    		    }
		class Pair1 implements Comparable<Pair1>{
				long price,rating;
				Pair1(long a,long b){
					this.price=a;
					this.rating=b;
				}
				@Override
				public int compareTo(Pair1 arg0) {
					
						return (int)(-1*((arg0.rating - arg0.price)-(rating-price)));
					
				}
			
			
			}
			  
			
