#include<bits/stdc++.h>
using namespace std;
#define int long long
const int MAXN = 3e5 + 5;
const int INF = 1e18;
void solve()
{
int n, m;
cin >> n >> m;
vector <int> a(n + 1), b(n + 1);
for (int i = 1; i <= n; i++) cin >> a[i];
for (int i = 1; i <= n; i++) cin >> b[i];
int l = 0, r = INF;
int ans = 0;
while (l <= r)
{
int mid = (l + r) / 2;
int cnt = 0;
for (int i = 1; i <= n; i++)
{
int cur = max(a[i], b[i]);
int need = (mid + cur - 1) / cur;
if (need <= m) cnt += need;
else
{
int free = cur * m;
int left = mid - free;
cnt += m;
cnt += (left + b[i] - 1) / b[i];
}
}
if (cnt <= n * m) l = mid + 1, ans = mid;
else r = mid - 1;
}
cout << ans;
}
signed main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
solve();
}
