#include <stdio.h>

const int Mod = 998244353;

int main()
{
	int N;
	char S[50001];
	scanf("%d", &N);
	scanf("%s", S);
	
	int i, j, k, l, cur, prev;
	long long dp[2][27][27] = {}, tmp;
	for (i = 0, cur = 1, prev = 0, dp[0][26][26] = 1; i < N; i++, cur ^= 1, prev ^= 1) {
		for (j = 0; j < 27; j++) {
			for (k = 0; k < 27; k++) {
				if (dp[prev][j][k] == 0) continue;
				tmp = dp[prev][j][k] % Mod;
				dp[prev][j][k] = 0;
				if (S[i] == '?') {
					for (l = 0; l < 26; l++) dp[cur][k][l] += tmp;
					if (j != 26) dp[cur][k][j] -= tmp;
					if (k != 26) dp[cur][k][k] -= tmp;
				} else if (S[i] != 'a' + j && S[i] != 'a' + k) dp[cur][k][S[i] - 'a'] += tmp;
			}
		}
	}
	
	long long ans = 0;
	for (i = 0; i < 27; i++) for (j = 0; j < 27; j++) ans += dp[prev][i][j];
	printf("%lld\n", ans % Mod);
	fflush(stdout);
	return 0;
}