#include <iostream>
#include <vector>
#include <algorithm>
#include <atcoder/modint>
using namespace std;
#define rep(i,n) for(int i=0; i<(n); i++)
using m32 = atcoder::modint998244353;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  int N; cin >> N;
  string S; cin >> S;
  
  m32 dp[27][27] = {};
  dp[26][26] = 1;
  for(char c : S){
    m32 buf[27][27] = {};
    rep(i,26){
      if(c != '?' && c != 'a' + i) continue;
      rep(j,27) rep(k,27) if(j!=i && k!=i) buf[k][i] += dp[j][k];
    }
    rep(i,27) rep(j,27) dp[i][j] = buf[i][j];
  }
  m32 ans = 0;
  rep(i,27) rep(j,27) ans += dp[i][j];
  cout << ans.val() << endl;
  return 0;
}