pub mod io {
    use std::io::{BufRead, ErrorKind};

    pub fn scan<R: BufRead>(r: &mut R) -> Vec<u8> {
        let mut res = Vec::new();
        loop {
            let buf = match r.fill_buf() {
                Ok(buf) => buf,
                Err(e) if e.kind() == ErrorKind::Interrupted => continue,
                Err(e) => panic!("{}", e), 
            };
            let (done, used, buf) = {
                match buf.iter().position(u8::is_ascii_whitespace) {
                    Some(i) => (i > 0 || res.len() > 0, i + 1, &buf[..i]),
                    None => (buf.is_empty(), buf.len(), buf),
                }
            };
            res.extend_from_slice(buf);
            r.consume(used);
            if done { return res; }
        }
    }

    #[macro_export]
    macro_rules! scan {
        ($r:expr, [$t:tt; $n:expr]) => {
            (0..$n).map(|_| scan!($r, $t)).collect::<Vec<_>>()
        };
        ($r:expr, [$t:tt]) => {
            scan!($r, [$t; scan!($r, usize)])
        };
        ($r:expr, ($($t:tt),*)) => {
            ($(scan!($r, $t)),*)
        };
        ($r:expr, Usize1) => {
            scan!($r, usize) - 1
        };
        ($r:expr, Bytes) => {
            io::scan($r)
        };
        ($r:expr, String) => {
            String::from_utf8(scan!($r, Bytes)).unwrap()
        };
        ($r:expr, $t:ty) => {
            scan!($r, String).parse::<$t>().unwrap()
        };
    }

    #[macro_export]
    macro_rules! input {
        ($($($v:ident)* : $t:tt),* $(,)?) => {
            let stdin = std::io::stdin();
            let ref mut reader = std::io::BufReader::new(stdin.lock());
            $(let $($v)* = scan!(reader, $t);)*
        };
    }
}

const MOD: u64 = 998_244_353;

fn main() {
    input! {
        _n: usize,
        s: Bytes,
    }

    let s = {
        s.iter()
            .map(|s| if *s == b'?' { 26 } else { (s - b'a') as usize })
            .collect::<Vec<_>>()
    };

    let mut dp = vec![0u64; 27 * 27];
    *dp.last_mut().unwrap() += 1;
    for s in s {
        let mut ndp = vec![0u64; 27 * 27];
        if s == 26 {
            let mut tmp = vec![0; 27];
            for (i, dp) in dp.iter().enumerate() {
                let ni = i % 27;
                tmp[ni] = (tmp[ni] + dp) % MOD;
                if i / 27 == 26 {
                    continue;
                }
                let ni = i % 27 * 27 + i / 27;
                ndp[ni] = (ndp[ni] + MOD - dp) % MOD;
            }
            for i in 0..27 {
                for j in 0..26 {
                    if i == j {
                        continue;
                    }
                    let ni = i * 27 + j;
                    ndp[ni] = (ndp[ni] + tmp[i]) % MOD;
                }
            }
        } else {
            for (i, dp) in dp.iter().enumerate() {
                if i / 27 == s || i % 27 == s {
                    continue;
                }    
                let ni = i % 27 * 27 + s;
                ndp[ni] = (ndp[ni] + dp) % MOD;
            }
        }
        dp = ndp;
    }

    let ans = {
        dp.iter()
            .enumerate()
            .filter(|&(i, _)| i / 27 != i % 27)
            .fold(0u64, |acc, (_, count)| (acc + count) % MOD)
    };
    println!("{}", ans);
}