#include <bits/stdc++.h>
#define ll long long
#define endl '\n'
using namespace std;
const int maxn=1e5+7;
ll n,k,ans=0;
int v[maxn],a[maxn];
int main() {
//	freopen("P9749_11.in","r",stdin);
	cin>>n>>k;
	v[1]=0;
	for(int i=2; i<=n; i++) {
		cin>>v[i];
		v[i]+=v[i-1];
	}
	for(int i=1; i<=n; i++)
		cin>>a[i];
	ll lasti=1,last=a[1],you=0;
	for(int i=2; i<=n; i++) {
		if(i!=n&&a[i]>=last) continue;
		ll lucheng=v[i]-v[lasti];
		ll sum=ceil(1.0*(lucheng-you)/k);
		ans+=sum*last;
		you=sum*k+you-lucheng;
		lasti=i,last=a[i];
	}
	cout<<ans<<endl;
	return 0;
}