#include <bits/stdc++.h>
#define uwu return 0;
using namespace std;
#define fs first
#define sc second
#define int long long
using ll = long long;
const ll SIZE = 3e5 + 5;
const long long INF = 2e18 + 5;
ll N;
ll M;
ll A[SIZE], B[SIZE], rm[SIZE];
bool eval(ll mn){
//cout << mn << '\n';
ll left_h = 0;
for (int i = 0; i < N;i++){
rm[i] = mn;
if((mn - 1LL) / A[i] + 1 < M){
rm[i] = 0LL;
left_h += M - ((mn - 1LL) / A[i] + 1LL);
}
else{
rm[i] -= M * A[i];
}
//cout << rm[i] << ' ';
}
//cout << left_h << '\n';
for (int i = 0; i < N;i++){
if(rm[i] <= 0LL) continue;
if(left_h < (rm[i] - 1) / B[i] + 1){
return false;
}
left_h -= ((rm[i] - 1LL) / B[i] + 1LL);
}
return true;
}
signed main(){
cin.tie(0), ios::sync_with_stdio(0);
cin >> N >> M;
for (int i = 0; i < N;i++){
cin >> A[i];
}
for (int i = 0; i < N;i++){
cin >> B[i];
if(B[i] > A[i])
A[i] = B[i];
}
ll L = 0, R = INF, m;
while(L < R){
m = (L + R + 1LL) >> 1;
if(eval(m))
L = m;
else
R = m - 1LL;
}
cout << L << '\n';
uwu
}
