import sys
input = sys.stdin.readline

mod=998244353

N,K=list(map(int,input().split()))
A=list(map(int,input().split()))
B=list(map(int,input().split()))

S=sum(B)

ANS=0

for i in range(N):
    ANS+=(S-B[i])*A[i]

ANS=ANS*(N//K-1)*pow(N-1,mod-2,mod)%mod

print(ANS)