#include <bits/stdc++.h>
#define int long long
#define double long double
#define pii pair <int,int>
#define tiii tuple <int, int, int>
#define f first
#define s second
#define all(x) x.begin(), x.end()
#define ub(a, b) upper_bound(a.begin(), a.end(), b) - a.begin()
#define lb(a, b) lower_bound(a.begin(), a.end(), b) - a.begin()
#define ve vector
#define graph(a, n) vector <int> a[n];
#define wgraph(a, n) vector <pii> a[n];
#define emb emplace_back
#define em emplace
#define ins insert
#define er erase
#define iShowSpeed cin.tie(NULL)->sync_with_stdio(false)
using namespace std;
template <typename T>
using greater_priority_queue = priority_queue<T, vector<T>, greater<T>>;
const int mod = 1e9 + 7;
const int inf = 1e18;
const int N = 3e5 + 5;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
int n, m, a[N], b[N];
int cal(int mid){
int use = 0;
for (int i = 1; i <= n; i++) {
if (a[i] > b[i]) {
if ((mid + a[i] - 1) / a[i] > m) {
use += m;
int need = mid - m * a[i];
use += (need + b[i] - 1) / b[i];
}
else use += (mid + a[i] - 1) / a[i];
}
else use += (mid + b[i] - 1) / b[i];
}
return use;
}
int32_t main(){
iShowSpeed;
cin >> n >> m;
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, ans = 0;
while (l <= r) {
int mid = (l + r) / 2;
if (cal(mid) > n * m) {
r = mid - 1;
}
else {
ans = mid;
l = mid + 1;
}
}
cout << ans;
}
