n,k=map(int,input().split())
M=998244353
d=n//k-1
A=list(map(int,input().split()))
B=list(map(int,input().split()))
ans=0
b=sum(B)
for i in range(n):
    ans+=d*pow(n-1,-1,M)*(b-B[i])*A[i]
    ans%=M
print(ans)