MOD = 998244353

def f(S):
    if S == "?":
        return -1
    else:
        return ord(S) - 97

n = int(input())
S = input()

if n == 1:
    if S == "?":
        print(26)
    else:
        print(1)
    exit()

S = list(map(f, S))
dp = [0] * (26 * 26)

if S[0] == -1:
    l1 = list(range(26))
else:
    l1 = [S[0]]

if S[1] == -1:
    l2 = list(range(26))
else:
    l2 = [S[1]]

for i in l1:
    for j in l2:
        if i != j:
            dp[i * 26 + j] = 1

for s in S[2:]:
    if s == -1:
        lst = list(range(26))
    else:
        lst = [s]
    ndp = [0] * (26 * 26)
    for i in range(26):
        for j in range(26):
            for k in lst:
                if i == k or j == k:
                    continue

                ndp[26 * j + k] += dp[26 * i + j]
                ndp[26 * j + k] %= MOD

    dp = ndp

print(sum(dp) % MOD)