#include<bits/stdc++.h>
using namespace std;
long long n,d,w[100005],v[100005],cost,l;
signed main()
{
	cin>>n>>d;
	for(long long i=1;i<n;i++)cin>>w[i];
	for(long long i=1;i<n;i++)cin>>v[i];
	for(long long i=2;i<n;i++)v[i]=min(v[i],v[i-1]);
	for(long long i=1;i<n;i++)
	{
		w[i]-=l;
		if(w[i]<=0)
		{
			l-=w[i];
			continue;
		}
		if(w[i]%d==0)
		{
			cost+=(w[i]/d)*v[i];
			l=0;
			continue;
		}
		cost+=(w[i]/d+1)*v[i];
		l=d-w[i]%d;
	}
	cout<<cost;
	return 0;
}