#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long int n, p;
  cin >> n >> p;
  string s;
  cin >> s;
  long long int i = n - 1;
  ++s[n - 1];
  while (i >= 0 && i < n) {
    if (s[i] - 97 >= p) {
      s[i] = 'a';
      i--;
      if (i >= 0) s[i]++;
      continue;
    }
    if ((i >= 1 && s[i] == s[i - 1]) || (i >= 2 && s[i] == s[i - 2])) {
      s[i]++;
    } else {
      i++;
    }
  }
  if (i == -1)
    cout << "NO" << endl;
  else
    cout << s << endl;
  return 0;
}
