import numpy as np

en2asc = lambda s: ord(s) - 97

N = int(input())
S = input()

MOD = 998244353

dp = np.zeros((27, 27), dtype=np.int64)
dp[-1, -1] = 1

for s in S:
    dp_sum = dp.sum(axis=1)
    new_dp = np.zeros((27, 27), dtype=np.int64)
    if s == '?':
        for i in range(26):
            tmp = dp_sum - dp[:, i]
            tmp[i] = 0
            new_dp[i] += tmp
    else:
        i = en2asc(s)
        tmp = dp_sum - dp[:, i]
        tmp[i] = 0
        new_dp[i] += tmp

    dp = new_dp % MOD

ans = dp.sum() % MOD
print(ans)