#include <bits/stdc++.h>   using namespace std;   const int maxn = 1e5 + 5;   int main() {     ios::sync_with_stdio(0);     cin.tie(0);     cout.tie(0);     int n, p;     string s;     bool f = 0;     cin >> n >> p >> s;     s =      + s;     for (int i = n + 1; i > 1; i--) {       for (char j = s[i] + 1; j <  a  + p; j++) {         if (j != s[i - 1] && j != s[i - 2]) {           f = 1;           s[i] = j;           for (int k = i + 1; k <= n + 1; k++) {             char ch =  a ;             for (; ch <  a  + p; ch++) {               if (ch != s[k - 1] && ch != s[k - 2]) break;             }             s[k] = ch;           }         }         if (f) break;       }       if (f) break;     }     if (f) {       for (int i = 2; i < s.size(); i++) cout << s[i];       cout <<   n ;     } else       cout <<  NO n ;   }   