#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
ll n,d,v[100005],a[100005],s,ans,p;
int main(){
	cin>>n>>d;
	for(ll i=1;i<=d;i++)cin>>v[i];
	for(ll i=1;i<=n;i++)cin>>a[i];
	ll now=1;
	while(now<n){
		ll tmp=now;
		while(tmp<n&&a[now]<=a[tmp])s+=v[tmp++];
		if(s>0){
			ll p=ceil(1.0*s/d);
			ans+=p*a[now];
			s-=d*p;
		}
		now=tmp;	
	}
	cout<<ans;
	return 0;
}