#include "bits/stdc++.h"

#define int long long

using namespace std;
using ll = long long;
using P = pair<ll, ll>;
const ll INF = (1LL << 61);
ll mod = 998244353;
struct mint {
	ll x; // typedef long long ll;
	mint(ll x = 0) :x((x%mod + mod) % mod) {}
	mint operator-() const { return mint(-x); }
	mint& operator+=(const mint a) {
		if ((x += a.x) >= mod) x -= mod;
		return *this;
	}
	mint& operator-=(const mint a) {
		if ((x += mod - a.x) >= mod) x -= mod;
		return *this;
	}
	mint& operator*=(const mint a) {
		(x *= a.x) %= mod;
		return *this;
	}
	mint operator+(const mint a) const {
		mint res(*this);
		return res += a;
	}
	mint operator-(const mint a) const {
		mint res(*this);
		return res -= a;
	}
	mint operator*(const mint a) const {
		mint res(*this);
		return res *= a;
	}
	mint pow(ll t) const {
		if (!t) return 1;
		mint a = pow(t >> 1);
		a *= a;
		if (t & 1) a *= *this;
		return a;
	}

	// for prime mod
	mint inv() const {
		return pow(mod - 2);
	}
	mint& operator/=(const mint a) {
		return (*this) *= a.inv();
	}
	mint operator/(const mint a) const {
		mint res(*this);
		return res /= a;
	}
};
istream& operator>>(istream& is, mint& a) { return is >> a.x; }
ostream& operator<<(ostream& os, const mint& a) { return os << a.x; }


signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	int N; cin >> N;
	string S; cin >> S;
	if (N == 1) {
		if (S == "?")cout << 26 << endl;
		else cout << 1 << endl;
		return 0;
	}
	bool ok = true;
	for (int i = 0; i < N; i++) {
		for (int j = i - 2; j <= i + 2; j++) {
			if (j < 0 || N <= j || i == j)continue;
			if (S[i] != '?' && S[i] == S[j])ok = false;
			
		}
	}
	if (!ok) {
		cout << 0 << endl; return 0;
	}
	vector<vector<mint>>dp(26, vector<mint>(26));
	for (int i = 0; i < 26; i++) {
		for (int j = 0; j < 26; j++) {
			if (i == j)continue;
			if (S[0] == '?' || (int)(S[0] - 'a') == i) {
				if (S[1] == '?' || (int)(S[1] - 'a') == j) {
					dp[i][j] = 1;
				}
			}
		}
	}
	for (int i = 2; i < N; i++) {
		vector<vector<mint>>now(26, vector<mint>(26, 0));
		for (int j = 0; j < 26; j++) {
			for (int k = 0; k < 26; k++) {
				if (j == k)continue;
				for (int t = 0; t < 26; t++) {
					if (j == t || k == t)continue;
					if (S[i] == '?' || S[i] - 'a' == t) {
						now[k][t] += dp[j][k];
					}
				}
			
			}
		}
		dp = now;
	}
	mint ans = 0;
	for (int i = 0; i < 26; i++) {
		for (int j = 0; j < 26; j++) {
			ans += dp[i][j];
		}
	}
	cout << ans << endl;
	return 0;
}