#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int N, p;
  cin >> N >> p;
  vector<char> str(N);
  for (char& c : str) {
    cin >> c;
    c -= 'a';
  }
  str.push_back(0);
  int idx = N - 1;
  while (0 <= idx && idx < N) {
    ++str[idx];
    if (str[idx] == p) {
      --idx;
    } else if ((idx - 2 < 0 || str[idx - 2] != str[idx]) &&
               (idx - 1 < 0 || str[idx - 1] != str[idx])) {
      str[++idx] = -1;
    }
  }
  if (idx == -1) {
    cout << "NO\n";
  } else {
    for (int i = 0; i < N; ++i) {
      str[i] += 'a';
      cout << str[i];
    }
  }
  return 0;
}
