#include <bits/stdc++.h>
using namespace std;
int n, p, i, ok, x, m, v[1005];
char ch[1005];
int main() {
  cin >> n >> p;
  cin >> ch;
  for (i = 1; i <= n; i++) {
    v[i] = ch[i - 1] - 'a' + 1;
  }
  ok = 1;
  for (i = n; i >= 2 && ok; i--) {
    x = v[i];
    v[i] = 0;
    x++;
    while (x == v[i - 1] || x == v[i - 2]) x++;
    if (x <= p) ok = 0;
  }
  if (ok == 0) {
    m = i + 1;
    v[m] = x;
    m++;
  } else if (ok && v[1] != p) {
    m = 2;
    v[1]++;
  } else {
    cout << "NO";
    return 0;
  }
  while (m <= n) {
    x = 1;
    while (x == v[m - 1] || x == v[m - 2]) x++;
    v[m] = x;
    m++;
  }
  for (i = 1; i <= n; i++) {
    ch[0] = v[i] + 'a' - 1;
    cout << ch[0];
  }
  return 0;
}
