#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000001


int main(){
	
	int n;
	cin>>n;
	
	string s;
	cin>>s;
	
	vector dp(27,vector<mint>(27,0));
	dp[26][26] = 1;
	
	rep(i,n){
		vector ndp(27,vector<mint>(27,0));
		
		rep(j,27){
			rep(k,27){
				if(dp[j][k]==0)continue;
				rep(l,26){
					if(l==j||l==k)continue;
					if(s[i] != '?' && s[i] != 'a' + l)continue;
					ndp[k][l] += dp[j][k];
				}
				
			}
		}
		swap(dp,ndp);
	}
	
	mint ans = 0;
	rep(i,27){
		rep(j,27)ans += dp[i][j];
	}
	
	cout<<ans.val()<<endl;
	
	return 0;
}