mod = 998244353


def rng(s):
    if s == "?":
        for i in range(26):
            yield i
    else:
        yield ord(s) - 97


N = int(input())
S = input()
dp = [[[0]*26 for _ in range(26)] for _ in range(N)]
for i in rng(S[0]):
    for j in rng(S[1]):
        if i == j:
            continue
        dp[1][i][j] = 1

for i in range(2, N):
    for x in rng(S[i]):
        for y in rng(S[i-1]):
            if x != y:
                for z in rng(S[i-2]):
                    if x != z:
                        dp[i][y][x] += dp[i-1][z][y]
                        dp[i][y][x] %= mod

ans = 0
for i in range(26):
    for j in range(26):
        ans += dp[N-1][i][j]
        ans %= mod
print(ans)