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