302 4
abdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcb
 300 3
abcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabc
 10 10
fajegfaicb
 70 4
dcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbd
 100 4
abcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabca
 5 5
aceba
 3 4
cdb
 2 2
ab
 11 3
abcabcabcab
 3 26
yzx
 2 4
cd
 7 26
zyxzyxz
 3 3
acb
 1 2
a
 77 7
cadgbagbcaecgfaegcdbeafbacbdfgaedgcdeabgebaecbeacgfebagedcegdafdgeacegfegfegf
 1 26
o
 10 3
cbacbacbac
 10 5
abcabcabca
 26 26
ahnxdnbfcriersyzdihuecojdi
 1 2
b
 13 7
gfegfegfegfeg
 1 1
a
 6 3
acbacb
 17 26
bazyxzyxzyxzyxzyx
 12 10
abcabcabcabc
 333 5
edcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedc
 1 26
z
 3 3
cab
 15 11
cgjkbadjfbdaikj
 6 3
abcabc
 17 4
dabcadcbdcadbcdbc
 30 7
cedcfedcfgcfgcbadcadgfaegfacgf
 30 26
abcabcabczyxzyxzyxzyxzyxzyxzyx
 2 2
ba

acbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbac
 acbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacb
 fajegfaicd
 NO
 abcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcd
 acebc
 dab
 ba
 acbacbacbac
 zab
 da
 NO
 bac
 b
 cadgbagbcaecgfaegcdbeafbacbdfgaedgcdeabgebaecbeacgfebagedcegdafdgeacfabcabcab
 p
 NO
 abcabcabcd
 ahnxdnbfcriersyzdihuecojdk
 NO
 NO
 NO
 bacbac
 bcabcabcabcabcabc
 abcabcabcabd
 NO
 NO
 cba
 cgjkbadjfbdajba
 acbacb
 dabcadcbdcadcabca
 cedcfedcfgcfgcbadcadgfaegfadba
 abcabcabdabcabcabcabcabcabcabc
 NO

import java.io.BufferedReader;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.HashMap;
import java.util.StringTokenizer;


public class Solution{
	
	///////////////////////////////////////////////////////////////////////////
	static class FastScanner{
		BufferedReader s;
		StringTokenizer st;
		
		public FastScanner(){
			st = new StringTokenizer("");
			s = new BufferedReader(new InputStreamReader(System.in));
		}
		
		public FastScanner(File f) throws FileNotFoundException{
			st = new StringTokenizer("");
			s = new BufferedReader (new FileReader(f));
		}
		
		public int nextInt() throws IOException{
			if(st.hasMoreTokens())
				return Integer.parseInt(st.nextToken());
			else{
				st = new StringTokenizer(s.readLine());
				return nextInt();
			}
		}
		
		public double nextDouble() throws IOException{
			if(st.hasMoreTokens())
				return Double.parseDouble(st.nextToken());
			else{
				st = new StringTokenizer(s.readLine());
				return nextDouble();
			}
		}
		
		public long nextLong() throws IOException{
			if(st.hasMoreTokens())
				return Long.parseLong(st.nextToken());
			else{
				st = new StringTokenizer(s.readLine());
				return nextLong();
			}
		}
		
		public String nextString() throws IOException{
			if(st.hasMoreTokens())
				return st.nextToken();
			else{
				st = new StringTokenizer(s.readLine());
				return nextString();
			}
			
		}
		public String readLine() throws IOException{
			return s.readLine();
		}
		
		public void close() throws IOException{
			s.close();
		}
		
	}
	
	////////////////////////////////////////////////////////////////////
	//		Number Theory
	
	long pow(long a,long b,long mod){
		long x = 1; long y = a;
		while(b > 0){
			if(b % 2 == 1){
				x = (x*y);
				x %= mod;
			}
			y = (y*y);
			y %= mod;
			b /= 2;			
		}
		return x;
	}
	
	int divisor(long x,long[] a){
	    long limit = x;
	    int numberOfDivisors = 0;

	    for (int i=1; i < limit; ++i) {
	        if (x % i == 0) {
	            limit = x / i;
	            if (limit != i) {
	                numberOfDivisors++;
	            }
	            numberOfDivisors++;
	        }
	    }
	    return numberOfDivisors;
    }
	
	void findSubsets(int array[]){
    	
	    long numOfSubsets = 1 << array.length; 
	    for(int i = 0; i < numOfSubsets; i++){
	    		
	    	int pos = array.length - 1;
	    	int bitmask = i;

	    	while(bitmask > 0){		
	    		if((bitmask & 1) == 1)
	   				ww.print(array[pos]+" ");
	   			bitmask >>= 1;
	        	pos--;
	    	}

	    	ww.println();
	   	}
	}
    
	
	public static int gcd(int a, int b){
		return b == 0 ? a : gcd(b,a%b);
	}
	
  
	public static int lcm(int a,int b, int c){
		return lcm(lcm(a,b),c);
	}
	
	public static int lcm(int a, int b){
		return a*b/gcd(a,b);
	}
	
	
	////////////////////////////////////////////////////////////////////
	
	
//	  private static  	FastScanner s = new FastScanner(new File("input.txt"));
//	  private static PrintWriter ww = new PrintWriter(new FileWriter("output.txt"));
	  private static FastScanner s = new FastScanner();   
      private static PrintWriter ww = new PrintWriter(new OutputStreamWriter(System.out));
//      private static Scanner s = new Scanner(System.in);
	@SuppressWarnings("unused")
	private static int[][] states = { {-1,0} , {1,0} , {0,-1} , {0,1} };
	
      
    //////////////////////////////////////////////////////////////////// 
      
     
    public static void main(String[] args) throws IOException{
 		new Solution().solve();
 	}
    
    ////////////////////////////////////////////////////////////////////
    
    int n;
    int p;
    char[] arr;
    char[] ans;
    
    boolean dfs(int i, boolean set){
    	
    	if(i == n) return true;
    	boolean status = false;
    	
    	for(char c = set ? 'a' : arr[i] ; c - 'a' < p ; c++){
    		
    		ans[i] = c;
    		if(i > 0 && ans[i] == ans[i-1]) continue;
    		if(i > 1 && ans[i] == ans[i-2]) continue;
    		status = dfs(i+1,set);
    		if(status){
    			return true;
    		}
    		set = true;
    	}
    	return false;
    }
    
	void solve() throws IOException{
		
		n = s.nextInt();
		p = s.nextInt();
		arr = s.nextString().toCharArray();
		ans = new char[n];
		
		arr[n-1]++;
		if(dfs(0,false)){
			for(int i=0;i<n;i++)
				ww.print(ans[i]);
			ww.println();
		}else ww.println("NO");
		
        s.close();
 	    ww.close(); 
     }
    
}