n=int(input())
s=["#"]+list(input())+["?","?"]
ans=1
mod=998244353
for i in range(1,n+1):
    if s[i]=="?":
        cnt=26
        if i>=2:cnt-=1
        if i>=3:cnt-=1
        if s[i+1]!="?":cnt-=1
        if s[i+2]!="?":cnt-=1
        ans*=cnt
        ans%=mod
    else:
        if i>=2 and s[i]==s[i-1]:ans=0
        if i>=3 and s[i]==s[i-2]:ans=0
print(ans)