#include <bits/stdc++.h>
using namespace std;
int n, p;
string s;
char last;
bool judge(int x) {
  string a = s;
  a[x]++;
  for (; a[x] <= last;) {
    if (x - 1 >= 0 && a[x] == a[x - 1])
      a[x]++;
    else if (x - 2 >= 0 && a[x] == a[x - 2])
      a[x]++;
    else
      break;
  }
  if (a[x] > last) return false;
  for (int i = x + 1; i < a.length(); i++) {
    a[i] = 'a';
    for (; a[i] <= last;) {
      if (i - 1 >= 0 && a[i] == a[i - 1])
        a[i]++;
      else if (i - 2 >= 0 && a[i] == a[i - 2])
        a[i]++;
      else
        break;
    }
    if (a[i] > last) return false;
  }
  cout << a << endl;
  return true;
}
int main() {
  ios_base::sync_with_stdio(false);
  while (cin >> n >> p) {
    cin >> s;
    int len = s.length();
    last = 'a' + p - 1;
    bool ok = false;
    for (int i = len - 1; i >= 0; i--) {
      if (judge(i)) {
        ok = true;
        break;
      }
    }
    if (!ok) {
      cout << "NO" << endl;
    }
  }
  return 0;
}
