#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 5;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n, p;
  string s;
  bool f = 0;
  cin >> n >> p >> s;
  s = "  " + s;
  for (int i = n + 1; i > 1; i--) {
    for (char j = s[i] + 1; j < 'a' + p; j++) {
      if (j != s[i - 1] && j != s[i - 2]) {
        f = 1;
        s[i] = j;
        for (int k = i + 1; k <= n + 1; k++) {
          char ch = 'a';
          for (; ch < 'a' + p; ch++) {
            if (ch != s[k - 1] && ch != s[k - 2]) break;
          }
          s[k] = ch;
        }
      }
      if (f) break;
    }
    if (f) break;
  }
  if (f) {
    for (int i = 2; i < s.size(); i++) cout << s[i];
    cout << "\n";
  } else
    cout << "NO\n";
}
