#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, i, p;
  char s[1003];
  cin >> n >> p;
  cin >> s;
  s[n - 1]++;
  i = n - 1;
  while (i < n && i >= 0) {
    if (s[i] - 'a' == p) {
      s[i] = 'a';
      i--;
      if (i >= 0) s[i]++;
    } else if (i >= 1 && (s[i - 1] == s[i] || s[i - 2] == s[i]))
      s[i]++;
    else
      i++;
  }
  if (i < 0)
    cout << "NO\n";
  else
    cout << s << endl;
  return 0;
}
