#include <bits/stdc++.h>   using namespace std;   const int M = 1000000007;   const int MM = 998244353;   const long double PI = acos(-1);   const long long INF = 2e18;   template <typename T, typename T1>   void amax(T &a, T1 b) {     if (b > a) a = b;   }   template <typename T, typename T1>   void amin(T &a, T1 b) {     if (b < a) a = b;   }   long long power(long long b, long long e) {     if (e == 0) return 1;     if (e & 1) return b * power(b * b, e / 2);     return power(b * b, e / 2);   }   long long power(long long b, long long e, long long m) {     if (e == 0) return 1;     if (e & 1) return b * power(b * b % m, e / 2, m) % m;     return power(b * b % m, e / 2, m);   }   long long modinv(long long a, long long m) { return power(a, m - 2, m); }   int TLE_TERROR() {     long long n, p, f = 0, zz = -1;     cin >> n >> p;     string s;     cin >> s;     if (p == 1) {       cout <<  NO ;       return 0;     }     if (p == 2) {       if (n > 2)         cout <<  NO ;       else if (n == 1) {         if (s[0] ==  a )           cout <<  b ;         else           cout <<  NO ;       } else {         if (s ==  ab )           cout <<  ba ;         else           cout <<  NO ;       }       return 0;     }     for (long long i = n - 1; i >= 0; i--) {       char z = s[i];       while ((z -  a  + 1) < p) {         z++;         if (i > 0 && z == s[i - 1]) continue;         if (i > 1 && z == s[i - 2]) continue;         long long q = n - i;         s[i] = z;         f = 1;         zz = i;         break;       }       if (f == 1) break;     }     if (f == 0)       cout <<  NO ;     else {       for (long long i = zz + 1; i < n; i++) {         if (i > 1) {           if (s[i - 1] !=  a  && s[i - 2] !=  a )             s[i] =  a ;           else if (s[i - 1] !=  b  && s[i - 2] !=  b )             s[i] =  b ;           else             s[i] =  c ;         } else {           if (s[i - 1] !=  a )             s[i] =  a ;           else if (s[i - 1] !=  b )             s[i] =  b ;           else             s[i] =  c ;         }       }       cout << s;     }     return 0;   }   int main() {     ios_base::sync_with_stdio(0);     cin.tie(NULL);     cout.tie(NULL);     long long int TESTS = 1;     while (TESTS--) TLE_TERROR();     return 0;   }   