#include<bits/stdc++.h>
using namespace std;
long long a,b,juli[10005],jiayou[10005],money[10005],zhuixiaojia=10005,qian;
int main(){
	cin>>a>>b;//表示公路上站点的数量和车每升油可以前进的距离
	for(long long i=1;i<a;i++){
		cin>>juli[i];
		jiayou[i]=juli[i];
	}//表示站点间的距离
	for(long long i=1;i<=a;i++){
		cin>>money[i];
	}//不同站点加油的价格
	for(long long i=1;i<a;i++){
		if(zhuixiaojia>money[i]){
			zhuixiaojia=money[i];
		}
		if(juli[i]%b==0){
			juli[i]-=b;
		}
		qian+=(juli[i]/b+1)*zhuixiaojia;
		if(i!=b){
			juli[i+1]-=((juli[i]/b+1)*b-jiayou[i]);
		}
	}cout<<qian;
	return 0;
}