#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
int n, m;
cin >> n >> m;
vector<ll> a(n), b(n);
for (int i = 0; i < n; ++i) cin >> a[i];
for (int i = 0; i < n; ++i) cin >> b[i];
vector<int> ord(n);
iota(begin(ord), end(ord), 0);
sort(begin(ord), end(ord), [&](int x, int y) {
return max(a[x], b[x]) > max(a[y], b[y]);
});
auto check = [&](ll need) -> bool {
ll free = 0;
//~ cerr << need << ":\n";
for (auto& i : ord) {
ll now = max(a[i], b[i]);
ll have = (need + now - 1) / now;
//~ cerr << now << " " << have << " " << free << "\n";
if (have <= m) {
free += m - have;
} else {
ll now_need = need - 1LL * m * now;
now = b[i];
ll add = (now_need + now - 1) / now;
if (add > free) return false;
free -= add;
}
}
return true;
};
ll left = 0, right = 1e18;
ll res = -1;
while (left <= right) {
ll m = (left + right) / 2;
if (check(m)) {
left = m + 1;
res = m;
} else {
right = m - 1;
}
}
cout << res << "\n";
}
