#include<bits/stdc++.h>
using namespace std;
const int N = 1000005;
int n,c,a[N],r[N];
int main()
{
	ios::sync_with_stdio(0);
	cin.tie(0); cout.tie(0);
	int ans = 0; 
	cin >> n >> c;
	for(int i=1; i<=n; i++) cin >> r[i];
	for(int i=1; i<=n; i++) cin >> a[i];
	for(int i=1; i<=n; i++)
	{
		for(int j=1; j<=n; j++)
		{
			if(a[i] >= r[j] && r[j] != 0)
			{
				a[i] -= r[j];
				r[j] = 0;
			}
			else if(r[j] >= a[i])
			{
				r[j] -= a[i];
				a[i] = 0;
			}
		}
		ans = ans+(a[i]*c);
	}
	cout << ans;
	return 0;
}