#include<bits/stdc++.h>
#include<atcoder/all>
using namespace std;
using namespace atcoder;
#define rep(i,n)for (int i = 0; i < int(n); ++i)
#define rrep(i,n)for (int i = int(n)-1; i >= 0; --i)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
template<class T> void chmax(T& a, const T& b) {a = max(a, b);}
template<class T> void chmin(T& a, const T& b) {a = min(a, b);}
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;
using mint = modint998244353;

mint dp[27][27], ndp[27][27];

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n;
    cin >> n;
    string s;
    cin >> s;
    dp[26][26] = 1;
    for(char si: s) {
        rep(i, 27) rep(j, 27) ndp[i][j] = 0;
        for(char c = 'a'; c <= 'z'; c++) if (si == c || si == '?') {
            int x = c - 'a';
            rep(i, 27) rep(j, 27) if (i != x && j != x) {
                ndp[j][x] += dp[i][j];
            }
        }
        rep(i, 27) rep(j, 27) dp[i][j] = ndp[i][j];
    }
    mint ans;
    rep(i, 27) rep(j, 27) ans += dp[i][j];
    cout << ans.val() << endl;
}