p9749

#include <iostream>
#include <cmath>
using namespace std;
  
const int N=100010;
  
long long int v[N],a[N];
  

int main()
{

    int n,d;
    cin>>n>>d;
    for(int i=0;i<n-1;++i)cin>>v[i];
    for(int i=0;i<n;++i)cin>>a[i];
    long long int minn=a[1];
    long long int s=0,ans=0;
    for(int i=0;i<n;++i)
    {
        minn=min(minn,a[i]);
        if(s<v[i])
        {
            long long o=(v[i]-s+d-1)/d;
            ans+=o*minn;
            s+=o*d;
        }
    }

    cout<<ans<<endl;
    return 0;
}