#include <bits/stdc++.h>
using namespace std;
#define int long long
#define ld long double
#define show(x,y) cout << y << " " << #x << endl;
#define show2(x,y,i,j) cout << y << " " << #x << " " << j << " " << #i << endl;
#define show3(x,y,i,j,p,q) cout << y << " " << #x << " " << j << " " << #i << " " << q << " " << #p << endl;
#define show4(x,y) for(auto it:y) cout << it << " "; cout << #x << endl;
typedef pair<int,int>pii;
typedef array<int,3>pi2;
void solve(){
int n,m;
cin >> n >> m;
pii arr[n];
for(int x=0;x<n;x++){
cin >> arr[x].first;
}
for(int x=0;x<n;x++){
cin >> arr[x].second;
arr[x].first=max(arr[x].first,arr[x].second);
}
int l=0;
int r=1e15;
int mid;
int best=0;
while(l<=r){
mid=(l+r)/2;
int counter=0;
for(int x=0;x<n;x++){
int hold=(mid+arr[x].first-1)/arr[x].first;
if(hold<=m){
counter+=hold;
}
else{
counter+=m;
int hold2=(mid-arr[x].first*m);
counter+=(hold2+arr[x].second-1)/arr[x].second;
}
}
if(counter<=n*m){
best=mid;
l=mid+1;
}
else r=mid-1;
}
cout << best;
}
int32_t main(){
ios::sync_with_stdio(0);
cin.tie(0);
int t=1;
//cin >> t;
//freopen("in.txt","r",stdin);
while(t--){
solve();
}
}
