#include <bits/stdc++.h>
using namespace std;
int n, p;
string s;
bool solve(int u) {
  if (u < 0) return false;
  s[u]++;
  if (s[u] - 'a' == p) {
    if (solve(u - 1)) {
      s[u] = 'a' - 1;
      return solve(u);
    }
  } else {
    if (u - 1 >= 0 && s[u] == s[u - 1]) return solve(u);
    if (u - 2 >= 0 && s[u] == s[u - 2]) return solve(u);
    return true;
  }
}
int main(void) {
  cin >> n >> p;
  cin >> s;
  if (solve(n - 1))
    cout << s << endl;
  else
    printf("NO\n");
  return 0;
}
