#include <bits/stdc++.h>
using namespace std;
template <class T>
void in(T &x) {
  x = 0;
  bool f = 0;
  char c = getchar();
  while (c < '0' || c > '9') {
    if (c == '-') f = 1;
    c = getchar();
  }
  while ('0' <= c && c <= '9') {
    x = (x << 3) + (x << 1) + (c ^ 48);
    c = getchar();
  }
  if (f) x = -x;
}
char s[1005];
int n, p;
char can(int x) {
  for (register int i = s[x] + 1; i <= 'a' + p - 1; ++i)
    if (i != s[x - 1] && i != s[x - 2]) return i;
  return 0;
}
signed main() {
  in(n), in(p);
  scanf("%s", s + 1);
  int pos = 0;
  for (register int i = n; i >= 1; --i) {
    if (can(i)) {
      s[i] = can(i);
      pos = i;
      break;
    }
  }
  if (!pos) {
    puts("NO");
    return 0;
  }
  for (register int i = pos + 1; i <= n; ++i) {
    for (register int j = 'a'; j <= 'a' + p - 1; ++j)
      if (j != s[i - 1] && j != s[i - 2]) {
        s[i] = j;
        break;
      }
  }
  for (register int i = 1; i <= n; ++i) putchar(s[i]);
  return 0;
}
