const MOD: usize = 998244353;

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<_>>();

    let mut dp = vec![vec![0usize; 27]; 27];
    dp[26][26] = 1;
    for i in 0..n {
        let mut next_dp = vec![vec![0usize; 27]; 27];
        if s[i] == '?' {
            let mut summary = vec![0usize; 27];
            for pprev in 0..27 {
                for prev in 0..27 {
                    if prev != 26 && pprev == prev { continue; }
                    summary[prev] += dp[prev][pprev];
                    summary[prev] %= MOD;
                }
            }
            for cur in 0..26 {
                for prev in 0..27 {
                    if cur == prev { continue; }
                    next_dp[cur][prev] += summary[prev];
                    next_dp[cur][prev] += MOD - dp[prev][cur];
                    next_dp[cur][prev] %= MOD;
                }
            }
        } else {
            let cur = s[i] as usize - 'a' as usize;
            for pprev in 0..=26 {
                if pprev == cur { continue; }
                for prev in 0..=26 {
                    if prev == cur { continue; }
                    next_dp[cur][prev] += dp[prev][pprev];
                    next_dp[cur][prev] %= MOD;
                }
            }
        }
        dp = next_dp;
    }
    println!("{}", dp.iter().map(|v| v.iter().sum::<usize>()).sum::<usize>() % MOD);
}