#include <bits/stdc++.h>
#define all(v) v.begin(), v.end()
#define sz(x) (int)x.size()
#define pb push_back
#define pii pair<int, int>
#define chmin(x, v) x = min(x, v)
#define chmax(x, v) x = max(x, v)
#define print(x) cout << #x << " est " << x << endl;
#define x first
#define y second
#define int long long
using namespace std;
const int N = 1e6;
int nCours, nWeeks;
int a[N], b[N];
bool isOk(int bInf){
int aAllouer = 0;
for (int iCours = 0; iCours < nCours; ++iCours){
int besoin = ceil(bInf/double(a[iCours]));
if (besoin <= nWeeks)
aAllouer += nWeeks - besoin;
else {
int reste = bInf - nWeeks * a[iCours];
reste = ceil(double(reste)/b[iCours]);
aAllouer -= reste;
}
}
// cout << aAllouer
return aAllouer >= 0;
}
signed main(){
cin >> nCours >> nWeeks;
for (int i = 0; i < nCours; ++i)
cin >> a[i];
for (int i = 0; i < nCours; ++i){
cin >> b[i];
chmax(a[i], b[i]);
}
int gauche = 0, droite = 1e18;
while (droite > gauche){
int mid = (gauche + droite)/2 + 1;
if (isOk(mid))
gauche = mid;
else
droite = mid - 1;
}
cout << gauche << endl;
return 0;
}
