#include <bits/stdc++.h>
using namespace std;
template <typename T>
T trace2(T a, T b) {
  cerr << a << " " << b << '\n';
}
template <typename T>
T trace3(T a, T b, T c) {
  cerr << a << " " << b << " " << c << '\n';
}
template <typename T>
T trace4(T a, T b, T c, T d) {
  cerr << a << " " << b << " " << c << " " << d << '\n';
}
template <typename T>
T trace5(T a, T b, T c, T d, T e) {
  cerr << a << " " << b << " " << c << " " << d << " " << e << '\n';
}
template <typename T>
T trace6(T a, T b, T c, T d, T e, T f) {
  cerr << a << " " << b << " " << c << " " << d << " " << e << " " << f << '\n';
}
void err(istream_iterator<string> it) {}
template <typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
  cerr << *it << " = " << a << '\n';
  err(++it, args...);
}
template <typename T>
T pow(T a, T b, long long int m) {
  T ans = 1;
  while (b > 0) {
    if (b % 2 == 1) ans = (ans * a) % m;
    b /= 2;
    a = (a * a) % m;
  }
  return ans % m;
}
void swap(long long int *a, long long int *b) {
  long long int temp = *a;
  *a = *b;
  *b = temp;
  return;
}
void fuck() {
  long long int n, p;
  cin >> n >> p;
  set<char> al;
  char ch = 'a';
  for (__typeof(p) i = (0) - ((0) > (p)); i != (p) - ((0) > (p));
       i += 1 - 2 * ((0) > (p))) {
    al.insert(ch);
    ++ch;
  }
  string s;
  cin >> s;
  long long int len = s.length();
  long long int idx = -1;
  for (long long int i = len - 1; i >= 0; --i) {
    char temp = s[i];
    for (auto c : al) {
      if (c > temp) {
        if ((i - 1 >= 0 && s[i - 1] == c) || (i - 2 >= 0 && s[i - 2] == c)) {
          continue;
        }
        s[i] = c;
        idx = i;
        break;
      }
    }
    if (idx != -1) break;
  }
  if (idx == -1) {
    cout << "NO";
    exit(0);
  }
  for (long long int i = idx + 1; i < len; ++i) {
    for (auto c : al) {
      if ((i - 1 >= 0 && s[i - 1] == c) || (i - 2 >= 0 && s[i - 2] == c)) {
        continue;
      }
      s[i] = c;
      break;
    }
  }
  cout << s << '\n';
  return;
}
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  fuck();
  return 0;
}
