#include <bits/stdc++.h>
using namespace std;
int n, p;
string s;
int main() {
  cin >> n >> p;
  cin >> s;
  int pos = n - 1;
  s[pos]++;
  while (pos < n && pos >= 0) {
    if (s[pos] - 'a' == p) {
      s[pos] = 'a';
      pos--;
      s[pos]++;
    } else {
      if (pos > 0 && s[pos] == s[pos - 1]) {
        s[pos]++;
      } else if (pos > 1 && s[pos] == s[pos - 2]) {
        s[pos]++;
      } else
        pos++;
    }
  }
  if (pos == -1)
    cout << "NO" << endl;
  else
    cout << s << endl;
}
