#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
#define ordered_set <int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>
#define ordered_multiset <int, null_type, less_equal <int>, rb_tree_tag, tree_order_statistics_node_update>
#define int long long
#define double long double
#define pii pair <int, int>
#define tiii tuple <int, int, int>
#define tiiii tuple <int, int, int, int>
#define emb emplace_back
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define iShowSpeed cin.tie(NULL)->sync_with_stdio(false)
#define matrix vector <vector <int>>
#define mat(n, m) vector <vector <int>> (n, vector <int> (m));
const int mod = 1e9 + 7;
const int inf = 1e18;
const matrix II = {{1, 0}, {0, 1}};
const int N = 2e5 + 5;
void solve(){
int n, m; cin >> n >> m;
vector <int> a(n + 1), b(n + 1);
for (int i = 1; i <= n; i++) cin >> a[i];
for (int i = 1; i <= n; i++) cin >> b[i];
int l = 0, r = inf;
int ans = 0;
while (l <= r) {
int mid = (l + r) / 2;
int cnt = 0;
for (int i = 1; i <= n; i++) {
int cur = max(a[i], b[i]);
int need = (mid + cur - 1) / cur;
if (need <= m) cnt += need;
else {
int free = cur * m;
int left = mid - free;
cnt += m;
cnt += (left + b[i] - 1) / b[i];
}
}
if (cnt <= n * m) l = mid + 1, ans = mid;
else r = mid - 1;
}
cout << ans;
}
int32_t main(){
iShowSpeed;
// int q; cin >> q; while (q--)
solve();
}
