#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
//#include<bits/extc++.h>
//__gnu_pbds
#define int ll
ll n,m;
vector<int> a;
vector<int> b;
bool check(ll k){
ll left = 0;
for(int i=0;i<n;i++){
ll c = k;
ll g = (c/a[i])+(c%a[i]!=0);
if(g>m) {
c-=(m*a[i]);
left -= (c/b[i])+(c%b[i]!=0);
}else{
left+=m-g;
}
}
return left>=0;
}
signed main(){
ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
cin>>n>>m;
a.resize(n);b.resize(n);
for(int i=0;i<n;i++) cin>>a[i];
for(int i=0;i<n;i++) {
cin>>b[i];
a[i] = max(a[i],b[i]);
}
ll l = 0; ll r = 1e18+5;
while(r-l>1){
ll mid = l+((r-l)/2);
if(check(mid)){
l = mid;
}else{
r = mid;
}
}
cout<<l<<"\n";
return 0;
}
