use std::io::*;
const MOD: i64 = 998244353;

fn main() {
    let mut s: String = String::new();
    std::io::stdin().read_to_string(&mut s).ok();
    let mut itr = s.trim().split_whitespace();
    let n: usize = itr.next().unwrap().parse().unwrap();
    let s: Vec<char> = itr.next().unwrap().chars().collect();

    let mut dp: Vec<Vec<Vec<i64>>> = vec![vec![vec![0; 27]; 27]; n + 1];
    dp[0][26][26] = 1;
    for i in 0..n {
        for j in 0..=26 {
            for k in 0..=26 {
                if s[i] == '?' {
                    for l in 0..26 {
                        if l == j || l == k {
                            continue;
                        }
                        dp[i + 1][l][j] += dp[i][j][k];
                        dp[i + 1][l][j] %= MOD;
                    }
                } else {
                    let c = (s[i] as u8 - 'a' as u8) as usize;
                    if c == j || c == k {
                        continue;
                    }
                    dp[i + 1][c][j] += dp[i][j][k];
                    dp[i + 1][c][j] %= MOD;
                }
            }
        }
    }

    let mut ans = 0i64;
    for i in 0..26 {
        for j in 0..26 {
            ans += dp[n][i][j];
            ans %= MOD;
        }
    }
    println!("{}", ans);
}