const MOD: usize = 998244353;
const SIZE: usize = 27;

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

    let mut dp = vec![vec![0usize; SIZE]; SIZE];
    dp[26][26] = 1;
    for i in 0..n {
        let mut next_dp = vec![vec![0usize; SIZE]; SIZE];
        if s[i] == '?' {
            let isum = (0..SIZE)
                .map(|prev| (0..SIZE).map(|pprev| dp[pprev][prev]).sum::<usize>() % MOD)
                .collect::<Vec<_>>();
            next_dp = (0..SIZE).map(|prev| {
                    (0..SIZE).map(|cur| {
                        if cur == prev || cur == 26 {
                            0
                        } else {
                            (MOD + isum[prev] - dp[cur][prev]) % MOD
                        }
                    })
                    .collect::<Vec<_>>()
                })
                .collect::<Vec<_>>();
        } else {
            let idx = s[i] as usize - 'a' as usize;
            for prev in 0..SIZE {
                if prev == idx { continue; }
                next_dp[prev][idx] = (0..SIZE)
                    .filter(|&pprev| pprev != idx)
                    .map(|pprev| dp[pprev][prev])
                    .sum::<usize>() % MOD;
            }
        }
        dp = next_dp;
    }
    println!("{}", dp.iter().map(|v| v.iter().sum::<usize>()).sum::<usize>() % MOD);
}