#include <bits/stdc++.h>
using namespace std;
int main(){
	ios::sync_with_stdio(0);
	cin.tie(0); cout.tie(0);
	int N; string S;
	cin >> N >> S;
	if(N == 1 && S == "?"){
		cout << 26 << "\n"; return 0; 
	}else if(N == 1){
		cout << 1 << "\n"; return 0; 
	}
	long long mod = 998244353;
	long long dp[50010][680] = {};
	long long ikkomae[50010][26] = {}; 
	for(int i = 0; i < 676; i++){
		char c1 = 'a' + (i / 26);
		char c2 = 'a' + (i % 26); 
		if(c1 == c2) continue; 
		if(S[0] != '?' && S[0] != c1) continue;
		if(S[1] != '?' && S[1] != c2) continue;
		dp[1][i] = 1;
		ikkomae[1][i % 26] ++; 
	}
	for(int i = 2; i < N; i++){
		for(int j = 0; j < 676; j++){
			char c1 = 'a' + (j / 26);
			char c2 = 'a' + (j % 26);
			if(c1 == c2) continue;
			if(S[i] != '?' && S[i] != c2) continue;
			dp[i][j] = (mod + ikkomae[i-1][j/26] - dp[i-1][26*(j%26)+j/26]) % mod; 
			ikkomae[i][j%26] += dp[i][j];
			ikkomae[i][j%26] %= mod; 
		}
	}
	long long ans = 0; 
	for(int i = 0; i < 676; i++) ans += dp[N-1][i];
	cout << ans % mod << "\n";
}