#include <bits/stdc++.h>
#define rep(i, l, r) for (int i = (l); i < (r); i++)
using namespace std;

typedef long long ll;

int main() {
    int N;
    string S;
    cin >> N >> S;
    if (N == 1) {
        if (S == "?") {
            cout << 26 << endl;
        } else {
            cout << 1 << endl;
        }
        return 0;
    }
    vector<vector<vector<ll>>> dp(2, vector<vector<ll>>(28, vector<ll>(28, 0)));
    ll MOD = 998244353, ans = 0;
    dp[0][26][27] = 1;
    rep(i, 0, S.size()) {
        rep(j, 0, 28) {
            rep(k, 0, 28) {
                dp[1 - i % 2][j][k] = 0;
            }
        }
        rep(j, 0, 28) {
            rep(k, 0, 28) {
                if (j == k) continue;
                rep(l, 0, 26) {
                    if (j == l || k == l) continue;
                    if (S[i] != '?' && S[i] - 'a' != l) continue;
                    dp[1 - i % 2][k][l] += dp[i % 2][j][k];
                    dp[1 - i % 2][k][l] %= MOD;
                }
            }
        }
        /*
        rep(j, 0, 28) {
            rep(k, 0, 28) {
                cout << dp[1 - i % 2][j][k] << " ";
            }
            cout << endl;
        }
        cout << endl;
        */
    }
    rep(i, 0, 26) {
        rep(j, 0, 26) {
            ans = (ans + dp[S.size() % 2][i][j]) % MOD;
        }
    }
    cout << ans << endl;
}