from _collections import defaultdict
n=int(input())
s=["#"]+list(input())

al=list("abcdefghijklmnopqrstuvwxyz")
dp=[defaultdict(int) for i in range(n+1)]
mod=998244353
dp[0][".","#"]=1

for i in range(n):
    for x,y in dp[i]:
        dp[i][x,y]%=mod
        if s[i+1]!="?":
            if s[i+1]==y or s[i+1]==x:continue
            dp[i+1][y,s[i+1]]+=dp[i][x,y]
            dp[i + 1][y, s[i + 1]]%=mod
        else:
            for z in al:
                if z!=x and z!=y:
                    dp[i+1][y,z]+=dp[i][x,y]
                    dp[i+1][y,z]%=mod

ans=0

for key in dp[n]:
    ans+=dp[n][key]
    ans%=mod
print(ans)