'''
    Python3(PyPy3) Template for Programming-Contest.

    author : sgsw

    generated : 2021/08/13   
    when : 22:01:19

'''

from collections import defaultdict
import sys


def input():
    return sys.stdin.readline().rstrip()


DXY = [(0, -1), (1, 0), (0, 1), (-1, 0)]  # LDRU
mod = 998244353
inf = 1 << 64
Alphabets = "abcdefghijklmnopqrstuvwxyz"


def is_Palindrome(s: str) -> bool:
    n = len(s)
    for i in range(n//2):
        if s[i] != s[n - 1 - i]:
            return False
    return True


def main():
    n = int(input())
    s = "$" + input()

    #2文字と3文字を含まなければおけ
    #したがって直前の二個を持てば良い

    dp = [defaultdict(int) for i in range(n + 1)]
    ans = 0

    if s[1] != "?":
        dp[1][s[0] + s[1]] = 1
    else:
        for char in Alphabets:
            dp[1][s[0] + char] = 1

    for i in range(2,n + 1):
        char = s[i]
        if char != "?":
            c = char
            for k,v in dp[i - 1].items():
                if is_Palindrome(k + c) == False and k[1] != c:
                    dp[i][k[1] + c] += v
                    dp[i][k[1] + c] %= mod 
        else:
            for c in Alphabets:
                for k,v in dp[i - 1].items():
                    if is_Palindrome(k + c) == False and k[1] != c:
                        dp[i][k[1] + c] += v
                        dp[i][k[1] + c] %= mod 
    #find answer
    for k, v in dp[n].items():
        ans += v
    ans %= mod
    print(ans)
    return 0


if __name__ == "__main__":
    main()