RT

#include<cstdio>
#include<cmath>
using namespace std;
long long dis[100010],cost[100010],n,d,f[100010];
double gas[100010];
int main()
{
	scanf("%lld%lld",&n,&d);
	for(int i=2;i<=n;i++) scanf("%lld",&dis[i]),dis[i]+=dis[i-1];
	for(int i=1;i<=n;i++) scanf("%lld",&cost[i]);
	long long lastmin=1;
	for(int i=2;i<=n;i++)
	{
		f[i]=f[lastmin]+ceil(((dis[i]-dis[lastmin])/(double)d)-gas[lastmin])*cost[lastmin];
		gas[i]=gas[lastmin]+ceil(((dis[i]-dis[lastmin])/(double)d)-gas[lastmin])-(dis[i]-dis[lastmin])/(double)d;
		if(cost[i]<cost[lastmin])
			lastmin=i;
	}
	printf("%lld\n",f[n]);
}