#include <bits/stdc++.h>
using namespace std;
const int maxn = 3e5 + 100;
long long int a[maxn], b[maxn], n, m;
void tl()
{
while (true)
{
int lksjfdj = 1;
}
}
bool check(long long int l)
{
long long int d = 0;
for (int i = 1;i <= n;i++)
{
long long int x = a[i] * m;
if (x > l)
{
d += (x - l) / a[i];
}
if (x < l)
{
if (b[i] == 0)
{
return 0;
}
d -= ((l - x + b[i] - 1) / b[i]);
}
}
return d >= 0;
}
int main()
{
cin >> n >> m;
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]);
}
long long int l = 0, r = 1e18 + 1;
while (r - l > 1)
{
long long int mid = (l + r) / 2;
if (check(mid))
{
l = mid;
}
else
{
r = mid;
}
}
cout << l;
}
