#include <bits/stdc++.h>
#define iter(a) a.begin(), a.end()
using namespace std;
typedef long long ll;
ll iceil(ll a, ll b){
return (a + b - 1) / b;
}
int n;
ll m;
vector<ll> a, b;
bool check(ll mn){
ll cnt = 0;
for(int i = 1; i <= n; i++){
ll need = iceil(mn, a[i]);
if(need <= m){
cnt += m - need;
continue;
}
ll tmp = mn - m * a[i];
cnt -= iceil(tmp, b[i]);
}
return cnt >= 0;
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
cin >> n >> m;
a.resize(n + 1);
b.resize(n + 1);
for(int i = 1; i <= n; i++) cin >> a[i];
for(int i = 1; i <= n; i++) cin >> b[i], a[i] = max(a[i], b[i]);
cout << *min_element(a.begin() + 1, a.end()) << "\n";
return 0;
ll l = 0, r = 1e15;
while(l < r){
ll mid = (l + r + 1) / 2;
if(check(mid)) l = mid;
else r = mid - 1;
}
cout << l << "\n";
return 0;
}
