n=int(input());mod=998244353
s=list(input())
dp=[[0]*27 for i in range(27)];dp[0][0]=1
for i in s:
    dp2=[[0]*27 for i in range(27)]
    if i=="?":
        dp3=[0]*27
        for j in range(27):
            for k in range(27):
                dp3[j]+=dp[j][k];dp3[j]%=mod
        for j in range(1,27):
            for l in range(27):
                if j==l:
                    continue
                dp2[j][l]+=dp3[l]-dp[l][j];dp2[j][l]%=mod
    else:
        j=ord(i)-96
        for l in range(27):
            if j==l:
                continue
            for k in range(27):
                if k!=j:
                    dp2[j][l]+=dp[l][k];dp2[j][l]%=mod
    dp=dp2
ans=0
for i in dp:
    for j in i:
        ans+=j
print(ans%mod)