n = int(input())
s = input()

mod = 998244353
dp = [[0] * 27 for _ in range(27)]
dp[-1][-1] = 1

for i in s:

    if i == "?":
        i_sum = [sum(dp[j][i] for j in range(27)) for i in range(27)]
        dp2 = [[0 if i == j or j == 26 else (i_sum[i] - dp[j][i]) % mod for j in range(27)]for i in range(27)]

    else:
        char = ord(i) - ord("a")

        dp2 = [[0] * 27 for _ in range(27)]
        for j in range(27):
            if j == char:
                continue

            dp[j][char] = sum(dp[i][j] for i in range(27) if i != char) % mod

    dp = dp2

print(sum(map(sum, dp)) % mod)