```#include<bits/stdc++.h>
using namespace std ;
const long long modd=1e9+7L; 
const int maxn=1e5+10;
typedef pair<long,long>P;
long long n,d,v[maxn],a[maxn],minn=1e18L,ans,you,yu,dis,cha;
int main()
{
	freopen("road.in","r",stdin);
	freopen("road.out","w",stdout); 
	cin>>n>>d;
	for(int i=1;i<n;i++)
	cin>>v[i];
	for(int i=1;i<=n;i++)
	cin>>a[i];
	for(int i=1;i<n;i++)
	{
		minn=min(minn,a[i]);dis-=v[i-1];
		cha=v[i]-dis;
		if(cha<0)continue;
		you=cha/d;yu=cha%d;
		if(yu!=0)you++;
		ans+=you*minn;dis+=you*d;
	}
	cout<<ans;
	return 0;

}

#include <bits/stdc++.h>
#define int long long
using namespace std;
int n, d;
int v[100000 + 5];
int a[100000 + 5];
signed main()
{
    freopen("road.in", "r", stdin);
    freopen("road.out", "w", stdout);
    
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin >> n >> d;
    for (int i = 1; i <= n - 1; i++)
        cin >> v[i];
    for (int i = 1; i <= n; i++)
        cin >> a[i];
    
    int ans = 0; // 总花费
    int minPrice = a[1]; // 最低油价
    int dis = 0; // 剩余里程
    for(int i = 1; i <= n - 1; i++)
    {
        minPrice = min(minPrice, a[i]);
        if (dis < v[i])
        {
            //剩余里程能跑多少跑多少
            v[i] -= dis;
            dis = 0;
        }
        else
        {
            //全都用剩余里程跑
            dis -= v[i];
            v[i] = 0;
        }
        //有剩余路程需要买油跑
        if (v[i])
        {
            //花钱买油
            ans += (v[i] / d + (v[i] % d != 0)) * minPrice;
            //剩余里程增加
            dis += (v[i] / d + (v[i] % d != 0)) * d;
            //跑
            dis -= v[i];
            v[i] = 0;
        }
    }

    cout << ans << "\n";
    return 0;
}