#include<bits/stdc++.h>
using namespace std;
int a[100000001],b[100000001],f[100000001],g[10000001],x[100000001];
int main()
{
    int n,d;
    cin>>n>>d;
    for(int i=2;i<=n;i++)  cin>>a[i];
    for(int i=1;i<=n;i++)  cin>>b[i];
    g[1]=b[1];
    for(int i=2;i<=n;i++)
    {
        g[i]=min(g[i-1],b[i]);
        a[i]-=x[i-1];
        x[i]=((a[i]-1)/d+1)*d-a[i];
        f[i]=f[i-1]+g[i-1]*((a[i]-1)/d+1);
    }
    cout<<f[n]<<endl;
    return 0;
}