#include <iostream>
#include <iomanip>
#include <algorithm>
#include <numeric>
#include <cmath>
#include <cassert>
#include <ctime>
#include <chrono>
#include <cstdio>
#include <random>
#include <vector>
#include <string>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <deque>
#include <queue>
#include <bitset>
#include <list>
#include <fstream>
#include <functional>
#include <complex>
using namespace std;
mt19937_64 rnd(chrono::steady_clock::now().time_since_epoch().count());
short skip_cin = 0;
const unsigned long long INF = 1e18, MOD = 1e9 + 7, MOD2 = 998244353, LOG = 19;
const long double EPS = 1e-9, PI = acos(-1);
bool check(unsigned long long n, unsigned long long m, vector<unsigned long long>& a, vector<unsigned long long>& b, unsigned long long nd)
{
unsigned long long lw = 0, gt = 0;
for (unsigned long long i = 0; i < n; i++)
{
if (a[i] * m > nd)
{
unsigned long long del = (a[i] * m - nd) / a[i];
gt += del;
}
}
for (unsigned long long i = 0; i < n; i++)
{
if (a[i] * m <= nd)
{
unsigned long long add = ((nd - a[i] * m) + b[i] - 1) / b[i];
if (add > n * m)
{
return false;
}
lw += add;
if (lw > gt)
{
return false;
}
}
}
return gt >= lw;
}
void solve()
{
unsigned long long n, m;
vector<unsigned long long> a, b;
cin >> n >> m;
a.resize(n);
b.resize(n);
for (unsigned long long i = 0; i < n; i++)
{
cin >> a[i];
}
for (unsigned long long i = 0; i < n; i++)
{
cin >> b[i];
a[i] = max(a[i], b[i]);
}
unsigned long long left = 0, right = (1ll << 60);
while (right - left > 1)
{
unsigned long long mid = (left + right) / 2;
if (check(n, m, a, b, mid))
{
left = mid;
}
else
{
right = mid;
}
}
cout << left << "\n";
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
srand(time(NULL));
int tst = 1;
//cin >> tst;
while (tst--)
{
solve();
}
return 0;
}
/*
<3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3
⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⠤⠖⠚⢉⣩⣭⡭⠛⠓⠲⠦⣄⡀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⢀⡴⠋⠁⠀⠀⠊⠀⠀⠀⠀⠀⠀⠀⠀⠀⠉⠳⢦⡀⠀⠀⠀⠀
⠀⠀⠀⠀⢀⡴⠃⢀⡴⢳⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠙⣆⠀⠀⠀
⠀⠀⠀⠀⡾⠁⣠⠋⠀⠈⢧⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⢧⠀⠀
⠀⠀⠀⣸⠁⢰⠃⠀⠀⠀⠈⢣⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⣇⠀
⠀⠀⠀⡇⠀⡾⡀⠀⠀⠀⠀⣀⣹⣆⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢹⠀
⠀⠀⢸⠃⢀⣇⡈⠀⠀⠀⠀⠀⠀⢀⡑⢄⡀⢀⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸⡇
⠀⠀⢸⠀⢻⡟⡻⢶⡆⠀⠀⠀⠀⡼⠟⡳⢿⣦⡑⢄⠀⠀⠀⠀⠀⠀⠀⠀⢸⡇
⠀⠀⣸⠀⢸⠃⡇⢀⠇⠀⠀⠀⠀⠀⡼⠀⠀⠈⣿⡗⠂⠀⠀⠀⠀⠀⠀⠀⢸⠁
⠀⠀⡏⠀⣼⠀⢳⠊⠀⠀⠀⠀⠀⠀⠱⣀⣀⠔⣸⠁⠀⠀⠀⠀⠀⠀⠀⢠⡟⠀
⠀⠀⡇⢀⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠠⠀⡇⠀⠀⠀⠀⠀⠀⠀⠀⢸⠃⠀
⠀⢸⠃⠘⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸⠁⠀⠀⢀⠀⠀⠀⠀⠀⣾⠀⠀
⠀⣸⠀⠀⠹⡄⠀⠀⠈⠁⠀⠀⠀⠀⠀⠀⠀⡞⠀⠀⠀⠸⠀⠀⠀⠀⠀⡇⠀⠀
⠀⡏⠀⠀⠀⠙⣆⠀⠀⠀⠀⠀⠀⠀⢀⣠⢶⡇⠀⠀⢰⡀⠀⠀⠀⠀⠀⡇⠀⠀
⢰⠇⡄⠀⠀⠀⡿⢣⣀⣀⣀⡤⠴⡞⠉⠀⢸⠀⠀⠀⣿⡇⠀⠀⠀⠀⠀⣧⠀⠀
⣸⠀⡇⠀⠀⠀⠀⠀⠀⠉⠀⠀⠀⢹⠀⠀⢸⠀⠀⢀⣿⠇⠀⠀⠀⠁⠀⢸⠀⠀
⣿⠀⡇⠀⠀⠀⠀⠀⢀⡤⠤⠶⠶⠾⠤⠄⢸⠀⡀⠸⣿⣀⠀⠀⠀⠀⠀⠈⣇⠀
⡇⠀⡇⠀⠀⡀⠀⡴⠋⠀⠀⠀⠀⠀⠀⠀⠸⡌⣵⡀⢳⡇⠀⠀⠀⠀⠀⠀⢹⡀
⡇⠀⠇⠀⠀⡇⡸⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠙⠮⢧⣀⣻⢂⠀⠀⠀⠀⠀⠀⢧
⣇⠀⢠⠀⠀⢳⠇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⡎⣆⠀⠀⠀⠀⠀⠘
⢻⠀⠈⠰⠀⢸⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠰⠘⢮⣧⡀⠀⠀⠀⠀
⠸⡆⠀⠀⠇⣾⠀⠀⠀⠀⠀⠀⠀⠀⠀⢠⠆⠀⠀⠀⠀⠀⠀⠀⠙⠳⣄⡀⢢⡀
<3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3
*/
