N = int(input())
S = input()
mod = 998244353
DP = [[[0] * 26 for _ in range(26)] for _ in range(N+1)]
DP[0][0][0]=1
for n in range(N):
    for i in range(26):
        for j in range(26):
            for k in range(26):
                if (i!=k or n<=1) and (j!=k or n==0) and (S[n]=='?' or ord(S[n])-97==k):
                    DP[n+1][k][j]=(DP[n+1][k][j]+DP[n][j][i])%mod
ans = 0
for i in range(26):
    for j in range(26):
        ans += DP[N][i][j]
print(ans%mod)