#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <iomanip>
#include <cmath>
#include <stdio.h>
#include <queue>
#include <deque>
#include <cstdio>
#include <set>
#include <map>
#include <bitset>
#include <stack>
#include <cctype>
using namespace std;
long long mod = 998244353;
long long dp[50050][30][30] = {};
long long sum1[30] = {};
int main() {
	long long n, co = 0;
	string s;
	cin >> n >> s;
	if (n == 1) {
		if (s == "?") {
			cout << 26 << endl;
		}
		else {
			cout << 1 << endl;
		}
		return 0;
	}
	for (int i = 0; i < 26; i++) {
		for (int j = 0; j < 26; j++) {
			int i1 = i, j1 = j;
			if (s[0] != '?') {
				i1 = s[0] - 'a';
			}
			if (s[1] != '?') {
				j1 = s[1] - 'a';
			}
			if (i1 != j1) {
				dp[2][j1][i1] = 1;
			}
		}
	}
	for (int j = 0; j < 26; j++) {
		for (int k = 0; k < 26; k++) {
			sum1[j] += dp[2][j][k];
		}
	}
	for (int i = 2; i < n; i++) {
		if (s[i] == '?') {
			for (int j = 0; j < 26; j++) {
				for (int k = 0; k < 26; k++) {
					if (j != k) {
						dp[i + 1][j][k] += sum1[k];
						dp[i + 1][j][k] -= dp[i][k][j];
					}
				}
			}
		}
		else {
			for (int j = 0; j < 26; j++) {
				if (s[i] - 'a' != j) {
					dp[i + 1][s[i] - 'a'][j] += sum1[j];
					dp[i + 1][s[i] - 'a'][j] -= dp[i][j][s[i] - 'a'];
				}
			}
		}
		for (int j = 0; j < 26; j++) {
			sum1[j] = 0;
			for (int k = 0; k < 26; k++) {
				dp[i + 1][j][k] %= mod;
				sum1[j] += dp[i + 1][j][k];
			}
		}
	}
	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;
}