#include <stdio.h>

#define MOD 998244353
#define MAX 50005

int dp[MAX][26][26];
char s[MAX];
int i, j, a, b, c, d, n, ans;

int main(void) {
    scanf("%d %s", &n, s);

    if (n == 1) {
        printf("%d\n", s[0] == '?' ? 26 : 1);
        return (0);
    }

    for (i = 0; i < 26; ++i) {
        for (j = 0; j < 26; ++j) {
            if (i == j) continue;
            if (s[0] != '?' && s[0] - 'a' != i) continue;
            if (s[1] != '?' && s[1] - 'a' != j) continue;

            ++dp[2][i][j];
        }
    }

    for (i = 2; i < n; ++i) {
        if (s[i] == '?') {
            for (a = 0; a < 26; ++a) {
                for (b = 0; b < 26; ++b) {
                    if (a == b) continue;
                    for (c = 0; c < 26; ++c) {
                        if (a == c || b == c) continue;
                        dp[i+1][b][c] += dp[i][a][b];
                        dp[i+1][b][c] %= MOD;
                    }
                }
            }
        } else {
            d = s[i] - 'a';
            for (a = 0; a < 26; ++a) {
                if (d == a) continue;
                for (b = 0; b < 26; ++b) {
                    if (a == b || d == b) continue;
                    dp[i+1][b][d] += dp[i][a][b];
                    dp[i+1][b][d] %= MOD;
                }
            }
        }
    }

    ans = 0;
    for (i = 0; i < 26; ++i) {
        for (j = 0; j < 26; ++j) {
            ans += dp[n][i][j];
            ans %= MOD;
        }
    }

    printf("%d\n", ans);
}