#include <bits/stdc++.h>
using namespace std;
void setIO(const string &name = "") {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  if (name.length()) {
    freopen((name + ".in").c_str(), "r", stdin);
    freopen((name + ".out").c_str(), "w", stdout);
  }
}
template <typename T>
void read(vector<T> &a) {
  for (auto &x : a) cin >> x;
}
template <typename T>
void read(vector<T> &a, long long n) {
  a.resize(n);
  for (auto &x : a) cin >> x;
}
template <typename T>
void print(vector<T> &a) {
  for (const auto &x : a) cout << x << ' ';
  cout << '\n';
}
void MOD(long long &x) {
  x %= 1000000007;
  if (x < 0) x += 1000000007;
}
long long gcd(long long a, long long b) { return !a ? b : gcd(b % a, a); }
long long n, p;
string s;
int32_t main() {
  setIO();
  cin >> n >> p;
  cin >> s;
  long long idx = n - 1;
  s[idx]++;
  while (idx >= 0 and idx < n) {
    if (s[idx] - 'a' >= p) {
      s[idx] = 'a';
      idx--;
      if (idx >= 0) s[idx]++;
    } else if (idx >= 1 and s[idx] == s[idx - 1])
      s[idx]++;
    else if (idx >= 2 and s[idx] == s[idx - 2])
      s[idx]++;
    else
      idx++;
  }
  if (idx < 0)
    cout << "NO\n";
  else
    cout << s << '\n';
  return 0;
}
