#include<bits/stdc++.h>
#define F first
#define S second
#define MAX 500005
#define oo 1e18
#define mod 1000000007
#define fast_in ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);cout.setf(ios::fixed);cout.precision(0);
using namespace std;
typedef long long ll;
#define pll pair<ll , ll>
#define vll vector<ll>
#define vvll vector<vll>
#define vpll vector<pll>
ll n,k;
pll a[MAX];
ll OK(ll x){
ll m=k*n,r=0;
for(int i=0;i<n;i++){
ll moves=min(k,(x+a[i].F-1)/a[i].F);
r+=(max((ll)0,x-(a[i].F*moves))+a[i].S-1)/a[i].S;
m-=moves;
}
if(r<=m)return 1;
return 0;
}
int main(){
fast_in
cin>>n>>k;
for(int i=0;i<n;i++){
cin>>a[i].F;
}
for(int i=0;i<n;i++){
cin>>a[i].S;a[i].F=max(a[i].S,a[i].F);
}
ll p=0,f=oo;
while(abs(p-f)!=1){
ll m=(p+f)/2;
if(OK(m)==1)p=m;
else f=m;
}
cout<<p;
return 0;
}
