#include <bits/stdc++.h>
using namespace std;
const long long MOD = 998244353;
int main(){
  int N;
  cin >> N;
  string S;
  cin >> S;
  if (N == 1){
    if (S[0] == '?'){
      cout << 26 << endl;
    } else {
      cout << 1 << endl;
    }
  } else {
    vector<vector<vector<long long>>> dp(N + 1, vector<vector<long long>>(26, vector<long long>(26, 0)));
    for (int i = 0; i < 26; i++){
      if (S[0] == '?' || S[0] - 'a' == i){
        for (int j = 0; j < 26; j++){
          if (S[1] == '?' || S[1] - 'a' == j){
            if (i != j){
              dp[2][i][j]++;
            }
          }
        }
      }
    }
    for (int i = 2; i < N; i++){
      for (int j = 0; j < 26; j++){
        for (int k = 0; k < 26; k++){
          for (int l = 0; l < 26; l++){
            if (S[i] == '?' || S[i] - 'a' == l){
              if (j != l && k != l){
                dp[i + 1][k][l] += dp[i][j][k];
                if (dp[i + 1][k][l] >= MOD){
                  dp[i + 1][k][l] -= MOD;
                }
              }
            }
          }
        }
      }
    }
    long long ans = 0;
    for (int i = 0; i < 26; i++){
      for (int j = 0; j < 26; j++){
        ans += dp[N][i][j];
      }
    }
    ans %= MOD;
    cout << ans << endl;
  }
}