#include <bits/stdc++.h>
using namespace std;
template <class T>
inline void smax(T &x, T y) {
  x = max((x), (y));
}
template <class T>
inline void smin(T &x, T y) {
  x = min((x), (y));
}
inline void scanInt(int &x) {
  register char c = getchar();
  x = 0;
  bool neg = false;
  while (c < 48 || c > 57) {
    if (c == '-') neg = true;
    c = getchar();
  }
  for (; c > 47 && c < 58; c = getchar()) {
    x = (x << 1) + (x << 3) + c - 48;
  }
  if (neg) x *= -1;
}
int n, m;
string origin;
string s;
bool place_char_start(int inx) {
  if (inx >= 2) {
    for (char i = s[inx] + 1; i < 'a' + m; i++)
      if (i != s[inx - 1] && i != s[inx - 2] && i != s[inx]) {
        s[inx] = i;
        return true;
      }
    return false;
  } else if (inx == 1) {
    for (char i = s[inx] + 1; i < 'a' + m; i++)
      if (i != s[inx - 1] && i != s[inx]) {
        s[inx] = i;
        return true;
      }
    return false;
  } else if (inx == 0) {
    s[0] = s[0] + 1;
    if (s[0] >= 'a' + m) return false;
    return true;
  }
  return false;
}
bool place_char(int inx) {
  if (inx >= 2) {
    for (char i = 'a'; i < 'a' + m; i++)
      if (i != s[inx - 1] && i != s[inx - 2]) {
        s[inx] = i;
        return true;
      }
    return false;
  } else if (inx == 1) {
    for (char i = 'a'; i < 'a' + m; i++)
      if (i != s[inx - 1]) {
        s[inx] = i;
        return true;
      }
  } else if (inx == 0) {
    s[0] = 'a';
    return true;
  }
  return false;
}
int main(int argc, const char *argv[]) {
  scanInt(n);
  scanInt(m);
  cin >> s;
  origin = s;
  for (int i = (int)s.size() - 1; i >= 0; i--) {
    s = origin;
    bool ok = true;
    ok &= place_char_start(i);
    for (int j = i + 1; ok && j < s.size(); j++) {
      if (place_char(j) == false) ok = false;
    }
    if (ok) {
      cout << s;
      return 0;
    }
  }
  cout << "NO";
  return 0;
}
