#include<bits/stdc++.h>
#define IOS ios_base::sync_with_stdio(false);cin.tie();cout.tie();
#define all(x) x.begin(), x.end()
#define int long long
#define pq priority_queue
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
#define pb push_back
#define pp pop_back
#define F first
#define S second
using namespace std;
int n, k;
int a[300010];
int b[300010];
bool check(int m, int c = 0) {
for (int i = 1; i <= n; i++) {
if (b[i] >= a[i]) {
int x = m / b[i];
if (b[i] * x < m) x++;
c += k - x;
}
else {
int x = m / a[i];
if (a[i] * x < m) x++;
if (x <= k) c += k - x;
else {
int r = (m - (a[i] * k));
int y = r / (a[i] - b[i]);
if ((a[i] - b[i]) * y < r) y++;
c -= y;
}
}
}
return c >= 0;
}
int search(int l, int r, int ans = 0) {
while (l < r) {
int m = (l + r) / 2;
if (check(m)) {ans = m; l = m + 1;}
else r = m - 1;
}
return ans;
}
void solve () {
cin >> n >> k;
for (int i = 1; i <= n; i++) cin >> a[i];
for (int i = 1; i <= n; i++) cin >> b[i];
cout << search(0, 1e9) << '\n';
}
signed main() {IOS solve(); return 0;}
