mod=998244353
n=int(input())
s=list(input())+["#"]

for i in range(n):
    s[i]=ord(s[i])-97

if n==1:
    if s[0]==-34:print(26)
    else:print(1)
    exit()

def _(i,j,k):
    return (j*27+k)*(n+2)+i

dp=[0]*(n+3)*900

if s[0]==s[1]==-34:
    for i in range(26):
        for j in range(26):
            if i!=j:dp[_(1,i,j)]+=1
elif s[0]==-34:
    for i in range(26):
        if i!=s[1]:dp[_(1,i,s[1])]+=1
elif s[1]==-34:
    for i in range(26):
        if i!=s[0]:dp[_(1,s[0],i)]+=1
else:
    if s[0]!=s[1]:dp[_(1,s[0],s[1])]+=1

sdp=[0]*30

for i in range(1,n):
    for j in range(26):
        for k in range(26):
            if j==k:continue
            if i>=2 and s[i]==-34:
                dp[_(i,j,k)]+=sdp[j]-dp[_(i-1,k,j)]
                dp[_(i,j,k)]%=mod
            if i==n-1:continue
            if s[i+1]!=-34:
                if s[i+1]==j or s[i+1]==k:continue
                dp[_(i+1,k,s[i+1])]+=dp[_(i,j,k)]
                dp[_(i + 1, k, s[i + 1])]%=mod
            else:
                pass
    for j in range(26):
        sdp[j]=sum(dp[_(i,k,j)] for k in range(26))%mod


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