#include <bits/stdc++.h>
using namespace std;
long long int bexp(long long int a, long long int b, long long int mod) {
  long long int res = 1;
  while (b) {
    if (b & 1) res = (res * a) % mod;
    a = (a * a) % mod;
    b >>= 1;
  }
  return res;
}
long long int gcd(long long int a, long long int b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
const long long int N = 100010;
const long long int mod = 1e9 + 7;
struct comp {
  bool operator()(const pair<long long int, long long int> a,
                  const pair<long long int, long long int> b) const {
    return a.second > b.second;
  }
};
int32_t main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.precision(10);
  auto chr = [&](long long int a) { return (char)('a' + a - 1); };
  long long int n, p;
  cin >> n >> p;
  string s;
  cin >> s;
  s += "  ";
  long long int f = 0;
  long long int pos = -1;
  for (long long int i = n - 1; i >= 0; --i) {
    if (s[i] == chr(p)) {
      s[i] = 'a';
    } else {
      s[i] = s[i] + 1;
      while ((i > 0 && s[i] == s[i - 1]) || (i > 1 && s[i] == s[i - 2]) ||
             s[i] == s[i + 1] || s[i] == s[i + 2]) {
        s[i] = s[i] + 1;
      }
      if (s[i] - 'a' >= p) {
        s[i] = 'a';
      } else {
        pos = i;
        f = 1;
        break;
      }
    }
  }
  if (f) {
    if (p == 2 && n - pos - 1 > 2) {
      cout << "NO\n";
      return 0;
    }
    for (long long int i = pos + 1; i < n; ++i) {
      long long int k = 1;
      char ch = chr(k);
      while ((i > 0 && ch == s[i - 1]) || (i > 1 && ch == s[i - 2])) {
        k = k % 3 + 1;
        ch = chr(k);
      }
      s[i] = ch;
    }
    cout << s << "\n";
  } else {
    cout << "NO\n";
  }
  return 0;
}
