#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, p;
  string ss;
  cin >> n >> p;
  cin >> ss;
  for (int i = n - 1; i >= 0; i--) {
    if (i - 2 >= 0) {
      for (int j = ss[i] - 'a' + 1; j < p; j++) {
        if (ss[i - 2] != j + 'a' && ss[i - 1] != j + 'a') {
          ss[i] = j + 'a';
          for (int k = i + 1; k < n; k++) {
            for (int l = 0; l < p; l++) {
              if (ss[k - 2] != l + 'a' && ss[k - 1] != l + 'a') {
                ss[k] = l + 'a';
                break;
              }
            }
          }
          cout << ss << endl;
          return 0;
        }
      }
    } else if (i == 0) {
      if (ss[i] - 'a' + 1 < p) {
        ss[i] = ss[i] + 1;
        for (int k = i + 1; k < n; k++) {
          for (int l = 0; l <= p; l++) {
            if (ss[k - 2] != l + 'a' && ss[k - 1] != l + 'a') {
              ss[k] = l + 'a';
              break;
            }
          }
        }
        cout << ss << endl;
      } else
        cout << "NO" << endl;
      return 0;
    } else {
      for (int j = ss[i] - 'a' + 1; j < p; j++) {
        if (ss[i - 1] != j + 'a') {
          ss[i] = j + 'a';
          for (int k = i + 1; k < n; k++) {
            for (int l = 0; l < p; l++) {
              if (ss[k - 2] != l + 'a' && ss[k - 1] != l + 'a') {
                ss[k] = l + 'a';
                break;
              }
            }
          }
          cout << ss << endl;
          return 0;
        }
      }
    }
  }
  return 0;
}
