#include <bits/stdc++.h>
using namespace std;
int stepletter(char *s, int n, int p) {
  for (int i = n; i >= 0; i--) {
    s[i] += 1;
    if (s[i] < 'a' + p) {
      return i;
    }
    s[i] = 'a';
  }
  return -1;
}
int main() {
  int n, p;
  char s[1001];
  char ans[1001] = {};
  cin >> n >> p;
  cin >> s;
  for (int i = 0; i < n; i++) {
    ans[i] = s[i];
  }
  int k = n - 1;
  while (1) {
    int j = stepletter(ans, k, p);
    if (j == -1) {
      cout << "NO" << endl;
      return 0;
    }
    for (; j <= k; j++) {
      if (j >= 1) {
        if (ans[j] == ans[j - 1]) break;
      }
      if (j >= 2) {
        if (ans[j] == ans[j - 2]) break;
      }
    }
    if (j <= k) {
      k = j;
      continue;
    }
    for (; j <= n - 1; j++) {
      if (j >= 2) {
        if (ans[j - 1] == 'a' || ans[j - 2] == 'a') {
          if (ans[j - 1] == 'b' || ans[j - 2] == 'b') {
            ans[j] = 'c';
          } else {
            ans[j] = 'b';
          }
        } else {
          ans[j] = 'a';
        }
      } else if (j = 1) {
        if (ans[j - 1] == 'a') {
          ans[j] = 'b';
        } else {
          ans[j] = 'a';
        }
      } else {
        ans[j] = 'a';
      }
    }
    break;
  }
  cout << ans << endl;
  return 0;
}
