#include<bits/stdc++.h>
using namespace std;
const long long mod=998244353;
int main(){
  long long N;
  string S;
  cin>>N>>S;
  vector<vector<long long>>dp(2,vector<long long>(26*27));
  if(S[0]=='?'){
    for(long long i=0;i<26;i++)dp[0][26*26+i]=1;
  }else{
    dp[0][26*26+int(S[0]-'a')]=1;
  }
  for(long long i=1;i<N;i++){
    vector<long long>moji1(26),moji2(27);
    for(long long j=0;j<26*27;j++){
      moji1[j%26]=(moji1[j%26]+dp[0][j])%mod;
      moji2[j/26]=(moji2[j/26]+dp[0][j])%mod;
      dp[1][j]=0;
    }
    if(S[i]=='?'){
      for(long long j=0;j<26*26;j++){
        if(j%26==j/26)continue;
        dp[1][j]=(moji1[j/26]-dp[0][(j%26)*26+(j/26)])%mod;
      }
    }else{
      for(long long j=0;j<26;j++){
        if(j==int(S[i]-'a'))continue;
        dp[1][j*26+int(S[i]-'a')]=(moji1[j]-dp[0][int(S[i]-'a')*26+j])%mod;
      }
    }
    for(long long j=0;j<26*27;j+=1){
      dp[0][j]=dp[1][j];
    }
  }
  long long ans=0;
  for(long long i=0;i<26*27;i++){
    ans=(ans+dp[0][i])%mod;
  }
  if(ans<0)ans+=mod;
  cout<<ans<<'\n';
}