#include<stdio.h>
#include<bits/stdc++.h>
#define N 100010
#define MOD 998244353
#define esp 1e-8
#define INF 999999999999999999
#define LL long long
#define rep(i,a,b,g) for(LL i=a;i<=b;i+=g)
#define rem(i,a,b,g) for(LL i=a;i>=b;i-=g)
#define repn(i,a,b,g) for(LL i=a;i<b;i+=g)
#define remn(i,a,b,g) for(LL i=a;i>b;i-=g)
#define pll pair<LL,LL>
#define mkp(x,y) make_pair(x,y)
#define i128 __int128
#define lowbit(x) ((x)&(-(x)))
#define lc (u<<1)
#define rc (u<<1|1)
using namespace std;
void read(i128 &x)
{
	i128 f=1;
	x=0;
	char ch=getchar();
	while(ch<'0'||ch>'9')
	{
		if(ch=='-')f=-1;
		ch=getchar();
	}
	while(ch>='0'&&ch<='9')
	{
		x=x*10+ch-'0';
		ch=getchar();
	}
	x*=f;
}
void write(i128 x)
{
	if(x>=10)write(x/10);
	putchar(x%10+'0');
}
LL n,d,v[N],a[N],lama=INF,dis,sum;
int main()
{
	cin>>n>>d;
	rep(i,1,n-1,1)cin>>v[i];
	rep(i,1,n,1)cin>>a[i];
	rep(i,1,n-1,1)
	{
		if(lama>=a[i])
		{
			lama=a[i];
		}
		if(dis<v[i])
		{
			LL t=v[i]-dis;
			t=(t/d)+((t%d)>0);
			sum+=t*lama;
			dis+=t*d;
			dis-=v[i];
		}
	}
	cout<<sum<<endl;
	return 0;
}