#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <cstring>
#include <algorithm>
#include <string>
#include <vector>
#include <stack>
#include <bitset>
#include <queue>
#include <set>
#include <unordered_set>
#include <map>
#include <unordered_map>

using namespace std;

#define IOS ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define in freopen("in.txt","r",stdin);
#define out freopen("out.txt","w",stdout);

typedef long long ll;
const ll MOD = 998244353;//1e9+7
const ll N = 1e5 + 10;
const ll MAXBIT = 60;
const ll INF = 1e18;
const double eps = 1e-8;

ll n,d;
ll v[N],a[N];
ll mn = INF; // min
vector<ll> next_min;
double gasoline;
ll ans;

int main() 
{
	cin >> n >> d;
	for(int i = 0;i < n - 1;i++) cin >> v[i];
	for(int i = 0;i < n;i++) 
	{
		cin >> a[i];
		if(a[i] < mn) 
		{
			mn = a[i];
			next_min.push_back(i);
		}
	}
	
	for(int i = 1,j = 0;i < (int)next_min.size();i++)
	{
		ll t = 0;
		for(int k = j + 1;k <= (int)next_min[i];k++) t += v[k-1];
		double x = ceil(t / d) + 1;
		if(x - gasoline > 0) ans += (x - gasoline) * a[j],gasoline = 0;
		else gasoline = gasoline - x;
		
//		cout<<next_min[i]<<" "<<t<<" "<<a[j]<<" "<<ans<<" "<<gasoline<<" "<<x<<endl;
		j = next_min[i];
	}
	cout<<ans;
	return 0;
}