#include<iostream>
#include<cmath>
using namespace std;
const int N=1e5+5;
long long n,d,v[N],a[N],ans,s,k,minn=0x3f3f3f3f;
int main()
{
	cin>>n>>d;
	for(int i=1;i<n;i++) cin>>v[i];
	for(int i=1;i<=n;i++) cin>>a[i];
	for(int i=1;i<n;i++)
	{
		if(minn>a[i]) 
		{
			if(minn!=0x3f3f3f3f)
            {
            	ans+=ceil(s/(d*1.0))*minn;
            	k=k+(ceil(s/(d*1.0))*d-s); s=0;
			}
			minn=min(minn,a[i]);
        }
        s+=v[i];
	}
	ans+=ceil(s/(d*1.0)-ceil(k/(d*1.0)))*minn;
	cout<<ans;
	return 0;
}

求求大佬看看哪错了，谢谢，会关注！！