#include <iostream>
#include <cmath>
using namespace std;
int main(){
    int n,d;
    cin>>n>>d;
    int youjia[n];
    int juli[n-1];
    long long kj=0;
    long long yuan=0;
        for (int i=0;i<n-1;i++){
        cin>>juli[i];
    }
    for (int i=0;i<n;i++){
        cin>>youjia[i];
    }
    int a=0;
    for (int i=0;i<n;i++){
        if (a>i){
            a-=1;
            continue;
        }
        for (int j=i+1;j<n;j++){
            if (youjia[i]<youjia[j]){
                yuan+=ceil((float)(juli[j-1]-kj)/d)*youjia[i];
                kj+=ceil((float)(juli[j-1]-kj)/d)*d;
            }
            else{
                yuan+=ceil((float)(juli[j-1]-kj)/d)*youjia[i];
                kj+=ceil((float)(juli[j-1]-kj)/d)*d;
                //cout<<kj<<" "<<yuan<<endl;
                kj-=juli[j-1];
                a=j;
                break;
            }
            //cout<<kj<<" "<<yuan<<endl;
            kj-=juli[j-1];
            a=j;
            if (j==n-1){
                break;
            }
        }
    }
    cout<<yuan;
    return 0;
}