#include<bits/stdc++.h>
#define int long long
#define all(v) v.begin(), v.end()
#define SZ(x) (int)x.size()
#define pii pair<int, int>
#define X first
#define Y second
using namespace std;
const int maxn = 2e5 + 10;
const int mod = 1e9 + 7;// 998244353;
const int llmx = 1e9;
void sol(){
int n, m;
cin >> n >> m;
vector< int > a(n), b(n);
for(auto &i : a) cin >> i;
for(auto &i : b) cin >> i;
for(int i = 0; i < n; ++i) a[i] = max(a[i], b[i]);
int l = 0, r = 1e18 + 7, ans = 0;
auto chk = [&](int t) -> bool {
int free = 0, need = 0;
for(int i = 0; i < n; ++i){
if(m * a[i] >= t){
free += m - (t + a[i] - 1) / a[i];
}
else{
need += (t - a[i] * m + b[i] - 1) / b[i];
}
if(need > n * m) return 0;
}
return free >= need;
};
while(l <= r){
int mid = (l + r) >> 1;
if(chk(mid)) l = mid + 1, ans = mid;
else r = mid - 1;
}
cout << ans << "\n";
}
/*
3 3
19 4 5
2 6 2
// 18
2 1
9 7
2 6
// 7
5 60000
630510219 369411957 874325200 990002527 567203997
438920902 634940661 593780254 315929832 420627496
// 41397427274960
4 25
1 2 3 4
1 2 3 4
// 48
*/
signed main(){
ios::sync_with_stdio(0), cin.tie(0), cerr.tie(0);
int t = 1; //cin >> t;
while(t--) sol();
}
