//#include <atcoder/all>
#include <bits/stdc++.h>
#include <cmath>
using namespace std;
//using namespace atcoder;
using ll = long long;
#define all(A) A.begin(),A.end()
using vll = vector<ll>;
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
using Graph = vector<vector<ll>>;



int main() {
	ll N;
	string S;
	ll mod=998244353;
	cin>>N>>S;
	vector<vector<vll>> DP(N+1,vector<vll> (26,vll(26,0)));
	if(N==1){
		if(S[0]=='?')cout<<26<<endl;
		else cout<<1<<endl;
		return 0;
	}
	if(S[0]=='?'){
		if(S[1]=='?'){
			rep(i,26)rep(j,26)
			{
				if(i==j)continue;
				DP[2][i][j]=1;
			}
		}
		else{
			rep(i,26){
				if(S[1]-'a'==i)continue;
				DP[2][i][S[1]-'a']=1;
			}
		}
	}
	else{
		if(S[1]=='?'){
			rep(i,26){
				if(S[0]-'a'==i)continue;
				DP[2][S[0]-'a'][i]=1;
			}
		}
		else{
			if(S[0]==S[1]){
				cout<<0<<endl;
				return 0;
			}
			DP[2][S[0]-'a'][S[1]-'a']=1;
		}
	}

	for(ll n=2;n<S.size();n++){
		if(S[n]=='?'){
			rep(i,26){
				rep(j,26){
					if(i==j)continue;
					rep(k,26){
						if(i==k||j==k)continue;
						DP[n+1][j][k]+=DP[n][i][j];
						DP[n+1][j][k]%=mod;
					}
				}
			}
		}
		else{
			rep(i,26){
				rep(j,26){
					if(i==j)continue;
					if(i==S[n]-'a')continue;
					if(j==S[n]-'a')continue;
					DP[n+1][j][S[n]-'a']+=DP[n][i][j];
					DP[n+1][j][S[n]-'a']%=mod;
				}
			}
		}

	}
	ll an=0;
	rep(i,26){
		rep(j,26){
			if(i==j)continue;
			an+=DP[N][i][j];
		}

	}
	cout<<an%mod<<endl;
	
}