import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		char[] s = sc.next().toCharArray();
		sc.close();

		if (n == 1) {
			if (s[0] == '?') {
				System.out.println(26);
			} else {
				System.out.println(1);
			}
			return;
		}

		int mod = 998244353;
		long[][][] dp = new long[n][26][26];
		for (int j1 = 0; j1 < 26; j1++) {
			if (s[0] != '?' && s[0] - 'a' != j1) {
				continue;
			}
			for (int j2 = 0; j2 < 26; j2++) {
				if (s[1] != '?' && s[1] - 'a' != j2) {
					continue;
				}
				if (j1 == j2) {
					continue;
				}
				dp[1][j1][j2] = 1;
			}
		}
		for (int i = 2; i < n; i++) {
			for (int c = 0; c < 26; c++) {
				if (s[i] != '?' && s[i] - 'a' != c) {
					continue;
				}
				for (int j1 = 0; j1 < 26; j1++) {
					if (j1 == c) {
						continue;
					}
					if (s[i - 1] != '?' && s[i - 1] - 'a' != j1) {
						continue;
					}
					for (int j2 = 0; j2 < 26; j2++) {
						if (j2 == c) {
							continue;
						}
						if (s[i - 2] != '?' && s[i - 2] - 'a' != j2) {
							continue;
						}
						dp[i][j1][c] += dp[i - 1][j2][j1];
					}
				}
			}
			for (int j1 = 0; j1 < 26; j1++) {
				for (int j2 = 0; j2 < 26; j2++) {
					dp[i][j1][j2] %= mod;
				}
			}
		}

		long ans = 0;
		for (int j1 = 0; j1 < 26; j1++) {
			if (s[n - 1] == '?') {
				for (int j2 = 0; j2 < 26; j2++) {
					ans += dp[n - 1][j1][j2];
				}
			} else {
				ans += dp[n - 1][j1][s[n - 1] - 'a'];
			}
		}
		ans %= mod;
		System.out.println(ans);
	}
}