#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const ll nx = 3e5+5;
ll n, m, a[nx], b[nx];
bool check(ll mid)
{
ll free = n * m, extra = 0;
// cout << "mid : " << mid << '\n';
for(int i=1;i<=n;i++)
{
ll mx = max(a[i], b[i]);
ll cnt = min(m, (mid + mx - 1) / mx);
free -= cnt;
ll remain = mid - cnt * mx;
if(remain > 0) extra += (remain + b[i] - 1) / b[i];
if(extra > n * m) return false;
}
// cout << "free : " << free << '\t' << "extra : " << extra << "\n\n";
return free >= extra;
}
int main()
{
cin.tie(NULL)->sync_with_stdio(false);
cin >> n >> m;
for(int i=1;i<=n;i++) cin >> a[i];
for(int i=1;i<=n;i++) cin >> b[i];
ll l = 0, r = 1e18;
while(l < r)
{
ll mid = (l + r + 1) / 2;
bool ok = check(mid);
if(ok) l = mid;
else r = mid - 1;
}
cout << l;
return 0;
}
