#include <bits/stdc++.h>
using namespace std;
long long int arr[1000000];
int main() {
  long long int n, p, i, j;
  cin >> n >> p;
  string s;
  cin >> s;
  for (i = n - 1; i >= 0; i--) {
    s[i]++;
    while ((s[i] - 'a') < p) {
      if (s[i] == s[i - 1] || s[i] == s[i - 2]) {
        s[i]++;
        continue;
      }
      for (j = i + 1; j < n; j++) {
        s[j] = 'a';
        while (s[j] == s[j - 1] || s[j] == s[j - 2]) s[j]++;
      }
      cout << s;
      exit(0);
    }
  }
  cout << "NO";
  return 0;
}
