#include <bits/stdc++.h>
using namespace std;
#define int long long
#define vi vector<int>
#define show3(x,y,z) cout<<#x<<": "<<x<<" | "<<#y<<": "<<y<<" | "<<#z<<": "<<z<<endl;
#define ld long double
void solve(){//nt n, int m, vector<int>&a, vector<int>&b){
int n,m;
cin>>n>>m;
vector<int> a(n),b(n);
for(auto &x:a)cin>>x;
for(auto &x:b)cin>>x;
int lo=1,hi=2e18;
while(lo<hi){
int mid=(lo+hi+1)/2; // min level is mid
int over=0;
for(int i=0;i<n;++i){
// try to get a level of >= mid
int take=0;
if(a[i]>b[i]){ // if its better to go for the course
take+=min(m,(int)ceil((ld)mid/a[i]));
}
// cout<<take<<endl;
int left=mid-take*a[i];
if(left>0){
// use as many b[i] to reach mid
take+=(ceil((ld)left/b[i]));
}
// cout<<take<<endl;
over+=m-take;
// show3(over,take,left);
}
// show3(mid,over,lo);
if(over<0)hi=mid-1;
else lo=mid;
}
cout<<lo;
// return lo;
}
int bf(int n, int m, vector<int>&a, vector<int>&b){
// int n,m;
// cin>>n>>m;
// vector<int> a(n),b(n);
// for(auto &x:a)cin>>x;
// for(auto &x:b)cin>>x;
int ans=LLONG_MAX;
for(int i=0;i<n;++i)ans=min(max(a[i],b[i]),ans);
// cout<<ans;
return ans;
}
mt19937_64 rnd(chrono::steady_clock::now().time_since_epoch().count());
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
solve();
// int m=1;
// for(int n=1;n<4;++n){
// cout<<"doing "<<n<<endl;
// for(int p=0;p<100000000;++p){
// vi a(n),b(n);
// for(auto &x:a)x=(rnd()%1'000'000'000+1);
// for(auto &x:b)x=(rnd()%1'000'000'000+1);
// if(solve(n,m,a,b)!=bf(n,m,a,b)){
// cout<<n<<endl;
// for(auto &x:a)cout<<x<<' ';cout<<endl;
// for(auto &x:b)cout<<x<<' ';cout<<endl;
// exit(0);
// }
// }
// }
// int n=2;
// vi a = {1111, 2111};
// vi b = {317, 81};
// solve(n,m,a,b);
// cout<<endl;
// bf(n,m,a,b);
// cout<<"\ndone";
}
