#include <bits/stdc++.h>
using namespace std;
#define int long long
#define pi pair<int, int>
#define vi vector<int>
#define vs vector<string>
#define vb vector<bool>
#define vpi vector<pi>
#define pb push_back
#define all(a) (a).begin(), (a).end()
const int mod = 1e9 + 7;
void solve()
{
int n, m;
cin >> n >> m;
int a[n], b[n];
for (int i = 0; i < n; i++)
{
cin >> a[i];
}
for (int i = 0; i < n; i++)
{
cin >> b[i];
}
int l = 0;
int r = 1e9;
auto ok = [&](int x)
{
int cnt = 0;
for (int i = 0; i < n; i++)
{
int mx = max(a[i], b[i]);
if (mx * m >= x)
{
cnt += (x / mx) + (x % mx != 0);
}
else
{
cnt += m;
cnt += ((x - (mx * m)) / b[i]) + ((x - (mx * m)) % b[i] != 0);
}
}
return cnt <= (n * m);
};
while (ok(r))
r *= 2;
while (l + 1 < r)
{
int m = (l + r) / 2;
if (ok(m))
{
l = m;
}
else
{
r = m;
}
}
cout << l << '\n';
}
signed main()
{
ios_base::sync_with_stdio(0);
cin.tie(NULL);
cout.tie(NULL);
int t = 1;
// cin >> t;
while (t--)
{
solve();
}
return 0;
}
