## https://yukicoder.me/problems/no/1646

MOD = 998244353

def is_palindrome(word):
    left = 0
    right = len(word) - 1
    while left < right:
        if word[left] != word[right]:
            return False
        left += 1
        right -= 1
    return True

def main():
    N = int(input())
    S = input()

    if len(S) == 1:
        if S[0] == "?":
            print(26)
        else:
            print(1)
        return
    
    if S[0] == "?":
        s1 = -1
    else:
        s1 = ord(S[0]) - ord("a")
    if S[1] == "?":
        s2 = -1
    else:
        s2 = ord(S[1]) - ord("a")
    
    dp = [[0] * 26 for _ in range(26)]
    for a in range(26):
        if s1 == -1 or s1 == a:
            for b in range(26):
                if s2 == -1 or s2 == b:
                    if a != b:
                        dp[b][a] = 1
    
    cum_dp = [0 for _ in range(26)]
    for b in range(26):
        for a in range(26):
            cum_dp[b] += dp[b][a]
            cum_dp[b] %= MOD
    
    for s in S[2:]:
        new_dp = [[0] * 26 for _ in range(26)]
        if s == "?":
            target = -1
        else:
            target = ord(s) - ord("a")

        for b in range(26):
            for c in range(26):
                if target == -1 or target == c:
                    if b != c:
                        new_dp[c][b] += (cum_dp[b] - dp[b][c]) % MOD
                        new_dp[c][b] %= MOD
        
        dp = new_dp
        cum_dp = [0 for _ in range(26)]
        for b in range(26):
            for a in range(26):
                cum_dp[b] += dp[b][a]
                cum_dp[b] %= MOD
    
    answer = 0
    for b in range(26):
        answer += cum_dp[b]
        answer %= MOD
    print(answer)


        
                






    






if __name__ == "__main__":
    main()