MOD = 998244353
N = int(input())
S = input()

num = lambda x:ord(x) - ord("a")

dp = [[[0]*27 for _ in range(27)] for _ in range(N+1)]
dp[0][26][26] = 1
for i in range(N):
    for j in range(27):

        SUM = 0
        for k in range(27):
            SUM += dp[i][j][k]
            SUM %= MOD

        if(S[i] == "?"):
            for l in range(26):
                if(l == j):continue
                dp[i+1][l][j] += SUM - dp[i][j][l]
                dp[i+1][l][j] %= MOD
        else:
            if(num(S[i]) == j):continue
            dp[i+1][num(S[i])][j] += SUM - dp[i][j][num(S[i])]
            dp[i+1][num(S[i])][j] %= MOD

ans = 0
for i in dp[-1]:
    ans += sum(i)
    ans %= MOD
print(ans)