en2asc = lambda s: ord(s) - 97

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

MOD = 998244353

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

for s in S:
    dp_sum = [sum(row) for row in dp]
    new_dp = [[0] * 27 for _ in range(27)]
    if s == '?':
        for i in range(26):
            for j in range(27):
                if i == j:
                    continue
                new_dp[i][j] += dp_sum[j] - dp[j][i]
    else:
        i = en2asc(s)
        for j in range(27):
            if i == j:
                continue
            new_dp[i][j] += dp_sum[j] - dp[j][i]

    dp = [[0] * 27 for _ in range(27)]
    for i in range(27):
        for j in range(27):
            dp[i][j] = new_dp[i][j] % MOD

ans = sum(sum(row) for row in dp) % MOD
print(ans)