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 {
            let mut kall = 0;
            for k in 0..=26 {
                kall += dp[i][j][k];
                kall %= MOD;
            }
            if s[i] == '?' {
                for l in 0..26 {
                    if l == j {
                        continue;
                    }
                    dp[i + 1][l][j] += (MOD + kall - dp[i][j][l]) % MOD;
                    dp[i + 1][l][j] %= MOD;
                }
            } else {
                let c = (s[i] as u8 - 'a' as u8) as usize;
                if c == j {
                    continue;
                }
                dp[i + 1][c][j] += (MOD + kall - dp[i][j][c]) % MOD;
                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);
}