#include <bits/stdc++.h>
using namespace std;

#define int long long

const int N=3e5+5;

int a[N],b[N],n,m;

bool check(int lim)
{
	int have=0,sumneed=0;
	for(int i=1;i<=n;i++)
	{
		int need=(lim-1)/a[i]+1;
		if(need<=m)
			have += m-need;
		else
			sumneed += (lim-a[i]*m-1)/b[i]+1;

		if(sumneed>1e18)
			return false;
	}
	return have>=sumneed;
}

signed main()
{
	cin >> n >> m;
	for(int i=1;i<=n;i++)
		scanf("%lld",&a[i]);

	for(int i=1;i<=n;i++)
	{
		scanf("%lld",&b[i]);
		a[i]=max(a[i],b[i]);
	}

	int l=1,r=1e18,ans=0;
	while(l<=r)
	{
		int mid=(l+r)/2;
		if(check(mid))
			ans=mid, l=mid+1;
		else
			r=mid-1;
	}
	printf("%lld\n",ans);
	return 0;
}