#include <bits/stdc++.h>
using namespace std;
const int N = 3e5 + 5;
using ll = long long;

ll n, m;
ll a[N], b[N], v[N];


bool check(ll x) {
	ll total = n * m;
	ll used = 0;
	for (int i = 1; i <= n; ++i) {
		ll m_vi = m * v[i];
		ll needed = 0;
		if (m_vi >= x) {
			needed = (x + v[i] - 1) / v[i];
		} else {
			ll rem = x - m_vi;
			needed = m + (rem + b[i] - 1) / b[i];
		}
		if (total - used < needed)
			return false;
		used += needed;
	}
	return used <= total;
}

int main() {

	ios::sync_with_stdio(false);
	cin.tie(0);

	cin >> n >> m;
	for (int i = 1; i <= n; ++i)
		cin >> a[i];
	for (int i = 1; i <= n; ++i) {
		cin >> b[i];
		v[i] = max(a[i], b[i]);
	}

	ll l = 0, r = 2e18;
	ll ans = 0;
	while (l <= r) {
		ll mid = l + (r - l) / 2;
		if (check(mid)) {
			ans = mid;
			l = mid + 1;
		} else {
			r = mid - 1;
		}
	}
	cout << ans << endl;
	return 0;
}