#include <bits/stdc++.h>
using namespace std;
const long long int M = 1000000007;
void solve() {
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  for (int i = n - 1; i >= 0; i--) {
    char cur = s[i];
    for (int pp = 1; pp < k; pp++) {
      cur++;
      if (cur - 'a' < k and cur - 'a' > 0 and (i - 1 < 0 or s[i - 1] != cur) and
          (i - 2 < 0 or s[i - 2] != cur)) {
        string temp = s;
        bool pos = 1;
        temp[i] = cur;
        for (int j = i + 1; j < n; j++) {
          char ch = 'a';
          if ((j - 1 >= 0 and temp[j - 1] == 'a') or
              (j - 2 >= 0 and temp[j - 2] == 'a'))
            ch = 'b';
          if (ch == 'b' and ((j - 1 >= 0 and temp[j - 1] == 'b') or
                             (j - 2 >= 0 and temp[j - 2] == 'b')))
            ch = 'c';
          if (ch - 'a' >= k) pos = 0;
          temp[j] = ch;
        }
        if (pos) {
          cout << temp << endl;
          return;
        }
      }
    }
  }
  cout << "NO\n";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int t = 1;
  while (t--) solve();
}
