/*
--------------              |   /
      |                     |  /
      |                     | /
      |             *       |/          |    |         ------            *
      |                     |           |    |        /      \
      |             |       |\          |    |       |       |\          |
   \  |             |       | \         |    |       |       | \         |
    \ |             |       |  \        |    |        \     /   \        |
     V              |       |   \        \__/|         -----     \       |
*/
#include <bits/stdc++.h>
using namespace std;
#define debug(x) cerr << "\e[1;31m" << #x << " = " << (x) << "\e[0m\n"
#define print(x) emilia_mata_tenshi(#x, begin(x), end(x))
template<typename T> void emilia_mata_tenshi(const char *s, T l, T r) {
	cerr << "\e[1;33m" << s << " = [";
	while(l != r) {
		cerr << *l;
		cerr << (++l == r ? ']' : ',');
	}
	cerr << "\e[0m\n";
}

#define EmiliaMyWife ios::sync_with_stdio(0); cin.tie(NULL);
using ll = int64_t;
using ull = uint64_t;
using ld = long double;
using uint = uint32_t;
const double EPS  = 1e-8;
const int INF     = 0x3F3F3F3F;
const ll LINF     = 4611686018427387903;
const int MOD     = 1e9+7;
/*--------------------------------------------------------------------------------------*/

signed main() { EmiliaMyWife
	int n;
	cin >> n;
	string s;
	cin >> s;
	vector<vector<vector<int>>> dp(n, vector<vector<int>>(26, vector<int>(27)));
	if(s[0] == '?') {
		for(int i = 0; i < 26; i++)
			dp[0][i][26] = 1;
	}
	else
		dp[0][s[0] - 'a'][26] = 1;
	const int mod = 998244353;
	for(int i = 1; i < n; i++) {
		for(int j = 0; j < 26; j++) {
			if(s[i] != '?' && s[i] - 'a' != j)
				continue;
			for(int k = 0; k < 26; k++) {
				if(j == k)
					continue;
				for(int l = 0; l < 27; l++) {
					if(j == l)
						continue;
					dp[i][j][k] = (dp[i][j][k] + dp[i - 1][k][l]) % mod;
				}
			}
		}
		//print(dp[i]);
	}
	int ans = 0;
	for(int i = 0; i < 26; i++)
		for(int j = 0; j < 27; j++)
			ans = (ans + dp[n - 1][i][j]) % mod;
	cout << ans << '\n';
}