#include <bits/stdc++.h>
using namespace std;

string p = "abcdefghijklmnopqrstuvwxyz";

int main(){
	int n;
	cin >> n;
	string s;
	cin >> s;
	int m = s.size();
	if (n == 1) {
		cout << "NO" << endl;
		return 0;
	}
	if (n == 2){
		if (s == "ab") cout << "ba" << endl;
		else cout << "NO" << endl;
		return 0;
	}
	
	string c;
	for (int i = 1; i <= m / 3; i++)
		c += p[n - 1], c += p[n - 2] ,c += p[n - 3];
	if (m % 3 == 1) c += p[n - 1];
	else if (m % 3 == 2) c += p[n - 1] ,c+= p[n - 2];
	if (c == s){
		cout << "NO" << endl;
		return 0;
	}
	//合法的情況
	set<string>st;
	for (int i = 0; i < n; i++){
		for (int j = 0; j < n; j++){
			for (int k = 0; k < n; k++){
				if (i == j || j == k || i == k) continue;
				string c;
				c += p[i], c += p[j], c += p[k];
				st.insert(c);
			}
		}
	}
	for (int i = m - 3; i >= 0; i -= 3){
		string t;
		t += s[i], t += s[i + 1], t += s[i + 2];
		auto g = st.find(t);
		g++;
		if (g == st.end()) continue;
		else{
			t = *g;
			s[i] = t[0], s[i + 1] = t[1], s[i + 2] = t[2];
			//----------------------------
			g = st.begin();
			t = *g;
			for (int j = i + 3; j < m; j++)
				s[j] = t[0], s[j + 1] = t[1], s[j + 2] = t[2];
			//----------------------------
			cout << s << endl;
			return 0; 
		}
	}
	if (m %3 == 1){
		if (s[0] - 'a' != n - 1){
			if (m == 1){
				s[0] += 1;
				cout << s << endl;
				return 0;
			}
			else{
				while (s[0] - 'a' != n - 1) {
					s[0] += 1;
					//--------------------------------
					int flag = 0;
					string g;
					for (auto it : st){
						g = it;
						if (m >= 2 && g[0] == s[0]) continue;
						if (m >= 3 && g[1] == s[0]) continue;
						cout << s[0];
						for (int i = 1; i <= m / 3; i++)
							cout << g;
						cout << endl;
						return 0;
					}
					//--------------------------------
				}
			}
		}
		cout << "NO" << endl;
		return 0;
	}
	if (m % 3 == 2){
		string t; t += p[n - 1], t += p[n - 2];
		string g; g += s[0], g += s[1];
		while (g != t){
			if (g[1] - 'a' != n - 1) g[1]++;
			else g[0] += 1, g[1] = 'a';
			if (g[0] - 'a' == n) break;
			if (g[0] == g[1]) continue;

			for (auto it : st){
				string h = it;
				if (m >= 3 && (g[0] == h[0] || g[1] == h[0])) continue;
				if (m >= 4 && (g[1] == h[2])) continue;
				s[0] = g[0], s[1] = g[1];
				cout << s[0] << s[1];
				for (int i = 1; i <= m / 3; i++)
					cout << h;
				return 0;
			}
		}
		cout << "NO" << endl;
	}
	return 0;
}