#include <bits/stdc++.h>
using namespace std;
int main() {
  long n, p;
  cin >> n >> p;
  string s;
  cin >> s;
  if (n == 1) {
    if (s[0] >= p + 'a' - 1) {
      cout << "NO" << endl;
      return 0;
    }
    s[0]++;
    cout << s << endl;
    return 0;
  }
  if (n == 2) {
    s[1]++;
    if (s[0] == s[1]) s[1]++;
    if (s[1] > p + 'a' - 1) {
      if (s[0] >= p + 'a' - 1) {
        cout << "NO" << endl;
        return 0;
      }
      s[0]++;
      s[1] = 'a';
    }
    cout << s << endl;
    return 0;
  }
  int i = n - 1;
  for (; i >= 2; i--) {
    int var = s[i] + 1;
    for (; s[i - 1] == var || s[i - 2] == var; var++)
      ;
    if (var <= p + 'a' - 1) break;
  }
  if (i == 1) {
    s[1]++;
    if (s[0] == s[1]) s[1]++;
    if (s[1] > p + 'a' - 1) {
      s[0] += 1;
      s[1] -= p - 1;
      if (s[0] > p + 'a' - 1) {
        cout << "NO" << endl;
        return 0;
      }
      s[1] = 'a';
    }
    i = 2;
    s[i] = 'a' - 1;
  }
  s[i]++;
  for (; s[i - 1] == s[i] || s[i - 2] == s[i]; s[i]++)
    ;
  i++;
  for (; i < n; i++) {
    s[i] = 'a';
    for (; s[i - 1] == s[i] || s[i - 2] == s[i]; s[i]++)
      ;
    if (s[i] > p + 'a' - 1) {
      cout << "NO" << endl;
      return 0;
    }
  }
  cout << s << endl;
  return 0;
}
