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

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

for i in s:
    dp2 = [[0] * 27 for _ in range(27)]

    if i == "?":
        for i in range(27):
            i_sum = sum(dp[j][i] for j in range(27))
            for j in range(26):
                if i == j:
                    continue

                dp2[i][j] += i_sum - dp[j][i]

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

        for i in range(27):
            for j in range(27):
                if i == char or j == char:
                    continue
                dp2[j][char] += dp[i][j]

    dp = [[i % mod for i in j] for j in dp2]

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