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.util.*;
import java.text.*;
import java.lang.*;
import java.math.BigInteger;
import java.util.regex.*;
public class Myclass {
		private static final int PriorityQueue = 0;
		/*public static ArrayList a[]=new ArrayList[200001];
		static boolean visited[]=new boolean [200001];
		static long value[];
		static long maxi[];
		static long dp[]=new long[200001];
		static long ans;
		static void dfs(pair n,int p)
		{
			visited[n.x]=true;
			dp[n.x]=Math.max(dp[p]+n.y,n.y);
			if(dp[n.x]>value[n.x])
			{
				ans++;
				return;
			}
			
			for(int i=0;i<a[n.x].size();i++)
			{
				pair y=(pair) a[n.x].get(i);
				if(!visited[y.x])
				{
					dfs(y,n.x);
				}
			}
		}*/
		static int n;
		static HashMap<Integer,Integer>hm=new HashMap<>();
		static long a[];
		static long b[],weight;

		static boolean f(double mid)
		{
			double total=mid;
			for(int i=0;i<n-1;i++)
			{
				total-=(total+weight)/a[i];
				if(total<=0)
					return false;
				total-=(total+weight)/b[i+1];
				if(total<=0)
					return false;
			}
			
			total-=(total+weight)/a[n-1];
			if(total<=0)
				return false;
			total-=(total+weight)/b[0];
			if(total<0)
				return false;
			return true;
		}
		static int type[];
		public static void main(String[] args) 
		{
		    InputReader in = new InputReader(System.in);
			PrintWriter pw = new PrintWriter(System.out);
			int n=in.nextInt();
			int m=in.nextInt();
			char s[]=in.nextLine().toCharArray();
			PriorityQueue<Integer>pq=new PriorityQueue<>();
			for(int i=0;i<n;i++)
			{
				pq.add(s[i]-'a'+1);
			}
			//pw.println(pq.toString());
			int pre=-100;
			int ans=0;
			while(!pq.isEmpty() && m!=0)
			{
				int i=pq.poll();
				if(pre+1<i)
				{
					pre=i;
					ans+=i;
					m--;
				}
			}
			if(m!=0)
				ans=-1;
			pw.println(ans);
			pw.flush();
			pw.close();
		}
		
		static class InputReader 
		{
		private final InputStream stream;
		private final byte[] buf = new byte[8192];
		private int curChar, snumChars;
		private SpaceCharFilter filter;
 
		public InputReader(InputStream stream) 
		{
			this.stream = stream;
		}
		public int snext() 
		{
			if (snumChars == -1)
				throw new InputMismatchException();
			if (curChar >= snumChars) 
			{
				curChar = 0;
				try 
				{
					snumChars = stream.read(buf);
				} 
				catch (IOException e) 
				{
					throw new InputMismatchException();
				}
				
				if (snumChars <= 0)
					return -1;
			}
			return buf[curChar++];
		}
 
		public int nextInt() 
	    {
			int c = snext();
			while (isSpaceChar(c)) 
			{
				c = snext();
			}
			int sgn = 1;
			if (c == '-')
		    {
				sgn = -1;
				c = snext();
			}
			int res = 0;
			do 
			{
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = snext();
			} while (!isSpaceChar(c));
			return res * sgn;
		}
 
		public long nextLong()
	    {
			int c = snext();
			while (isSpaceChar(c)) 
			{
				c = snext();
			}
			int sgn = 1;
			if (c == '-') 
			{
				sgn = -1;
				c = snext();
			}
			long res = 0;
			do 
			{
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = snext();
			} while (!isSpaceChar(c));
			return res * sgn;
		}
 
		public int[] nextIntArray(int n) 
		{
			int a[] = new int[n];
			for (int i = 0; i < n; i++) 
			{
				a[i] = nextInt();
			}
			return a;
		}
		static class tri implements Comparable<tri> {
			int p, c, l;
 
			tri(int p, int c, int l) {
				this.p = p;
				this.c = c;
				this.l = l;
			}
 
			public int compareTo(tri o) {
				return Integer.compare(l, o.l);
			}
		}
 
		public String readString()
	    {
			int c = snext();
			while (isSpaceChar(c)) 
			{
				c = snext();
			}
			StringBuilder res = new StringBuilder();
			do 
			{
				res.appendCodePoint(c);
				c = snext();
			} while (!isSpaceChar(c));
			return res.toString();
		}
 
		public String nextLine() 
		{
			int c = snext();
			while (isSpaceChar(c))
				c = snext();
			StringBuilder res = new StringBuilder();
			do 
			{
				res.appendCodePoint(c);
				c = snext();
			} while (!isEndOfLine(c));
			return res.toString();
		}
 
		public boolean isSpaceChar(int c) 
		{
			if (filter != null)
				return filter.isSpaceChar(c);
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}
 
		private boolean isEndOfLine(int c) 
		{
			return c == '\n' || c == '\r' || c == -1;
		}
 
		public interface SpaceCharFilter
	    {
			public boolean isSpaceChar(int ch);
		}
	}
		public static long mod = 1000000007;
		public static int d;
		public static int p;
		public static int q;
		
		public static int[] suffle(int[] a,Random gen)
		{
			int n = a.length;
			for(int i=0;i<n;i++)
			{
				int ind = gen.nextInt(n-i)+i;
				int temp = a[ind];
				a[ind] = a[i];
				a[i] = temp;
			}
			return a;
		}
		
		public static void swap(int a, int b){
			int temp = a;
			a = b;
			b = temp;
		}
		
		public static HashSet<Integer> primeFactorization(int n)
		{
			HashSet<Integer> a =new HashSet<Integer>();
			for(int i=2;i*i<=n;i++)
			{
				while(n%i==0)
				{
					a.add(i);
					n/=i;
				}
			}
			if(n!=1)
				a.add(n);
			return a;
		}
		
		public static void sieve(boolean[] isPrime,int n)
		{
			for(int i=1;i<n;i++)
				isPrime[i] = true;
			
			isPrime[0] = false;
			isPrime[1] = false;
			
			for(int i=2;i*i<n;i++)
			{
				if(isPrime[i] == true)
				{
					for(int j=(2*i);j<n;j+=i)
						isPrime[j] = false;
				}
			}
		}
		
		public static int GCD(int a,int b)
		{
			if(b==0)
				return a;
			else
				return GCD(b,a%b);
		}
		
		public static long GCD(long a,long b)
		{
			if(b==0)
				return a;
			else
				return GCD(b,a%b);
		}
		
		public static void extendedEuclid(int A,int B)
		{
			if(B==0)
			{
				d = A;
				p = 1 ;
				q = 0;
			}
			else
			{
				extendedEuclid(B, A%B);
				int temp = p;
				p = q;
				q = temp - (A/B)*q;
			}
		}
		
		public static long LCM(long a,long b)
		{
			return (a*b)/GCD(a,b);
		}
		
		public static int LCM(int a,int b)
		{
			return (a*b)/GCD(a,b);
		}
		
		public static int binaryExponentiation(int x,int n)
		{
		    int result=1;
		    while(n>0)
		    {
		        if(n % 2 ==1)
		            result=result * x;
		        x=x*x;
		        n=n/2;
		    }
		    return result;
		}
		
		public static long binaryExponentiation(long x,long n)
		{
		    long result=1;
		    while(n>0)
		    {
		        if(n % 2 ==1)
		            result=result * x;
		        x=x*x;
		        n=n/2;
		    }
		    return result;
		}
		
		public static int modularExponentiation(int x,int n,int M)
		{
		    int result=1;
		    while(n>0)
		    {
		        if(n % 2 ==1)
		            result=(result * x)%M;
		        x=(x%M*x)%M;
		        n=n/2;
		    }
		    return result;
		}
		
		public static long modularExponentiation(long x,long n,long M)
		{
		    long result=1;
		    while(n>0)
		    {
		        if(n % 2 ==1)
		            result=(result%M * x%M)%M;
		        x=(x%M * x%M)%M;
		        n=n/2;
		    }
		    return result;
		}
		
		public static long modInverse(int A,int M)
		{
		    return modularExponentiation(A,M-2,M);
		}
		
		public static long modInverse(long A,long M)
		{
		    return modularExponentiation(A,M-2,M);
		}
		
		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;
		}
		
		 public static long[] shuffle(long[] a, Random gen){ 
		        for(int i = 0, n = a.length;i < n;i++){ 
		            int ind = gen.nextInt(n-i)+i; 
		            long d = a[i]; 
		            a[i] = a[ind]; 
		            a[ind] = d; 
		        } 
		        return a; 
		    }
		static class pair implements Comparable<pair>
		{
			Integer x;
			Long y;
			pair(int a,long m)
			{
				this.x=a;
				this.y=m;
			}
 
			
 
 
		public boolean isEmpty() {
				// TODO Auto-generated method stub
				return false;
			}
 
 
 
 
 
		public int compareTo(pair o) {
				int result = x.compareTo(o.x);
				if(result==0)
					result = y.compareTo(o.y);
				
			return result;
			}  
			
			public String toString()
			{
				return x+" "+y;
			}
			
			public boolean equals(Object o)
			{
				if (o instanceof pair)
			    {
					pair p = (pair)o;
					return p.x == x && p.y == y ;
				}
				return false;
			}
			
			public int hashCode()
			{
				return new Long(x).hashCode()*31 + new Long(y).hashCode();
			}
		}
		static  class triplet
		{
			Long x;
			Long y;
			Long z;
			triplet(long x,long y,long z)
			{
				this.x=x;
				this.y=y;
				this.z=z;
			}
		}
		static class jodi
		{
			Integer x;
			Integer y;
			jodi(int x,int y)
			{
				this.x=x;
				this.y=y;
			}
		}
}
