#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  long long int n = 0, p = 0;
  string s;
  cin >> n >> p;
  char maxC = 'a' + p, temp;
  cin >> s;
  for (long long int i = s.length() - 1; i >= 0; i--) {
    if ((maxC - s[i] - 1) > 0) {
      bool flag = true;
      temp = s[i] + 1;
      while (flag && temp < maxC) {
        flag = false;
        if ((i - 1) >= 0 && temp == s[i - 1]) flag = true;
        if (i - 2 >= 0 && temp == s[i - 2]) flag = true;
        if (flag) temp++;
      }
      if (!flag) {
        s[i] = temp;
        for (long long int j = i + 1; j < s.length(); j++) {
          char m = 'a' - 1;
          bool flag2 = true;
          while (flag2 && m < maxC) {
            m++;
            flag2 = false;
            if ((j - 1) >= 0 && m == s[j - 1]) flag2 = true;
            if ((j - 2) >= 0 && m == s[j - 2]) flag2 = true;
          }
          if (!flag2) {
            s[j] = m;
          }
        }
        cout << s;
        return 0;
      }
    }
  }
  cout << "NO";
  return 0;
}
