#include <bits/stdc++.h>
#define fio ios_base::sync_with_stdio(0);cin.tie(0);
#define ll long long
#define pb push_back
#define fi first
#define se second
#define en exit(0);
using namespace std;
const int N = 3e5 + 5;
int a[N], b[N];
bool can(ll mn, int n, int m)
{
ll need = 0, have = 1ll * n * m;
// need - how many b[i] choices we need
// have - how many b[i] choices we have left
for(int i = 1;i <= n;i++)
{
if(a[i] > b[i])
{
ll op = (mn + a[i] - 1) / a[i];
// op - how many a[i]'s to have taken >= mn in total
ll take = min(1ll * m, op);
//take - can't take more than m a[i]'s
have -= take;
ll lft = max(0ll, mn - 1ll * a[i] * take);
// lft - how much we have left after taking the a[i]'s
need += (lft + b[i] - 1) / b[i];
}
else
need += (mn + b[i] - 1) / b[i];
if(need > n * m)
return 0;
}
return (have >= need);
}
int main()
{
fio
// ifstream cin("in.in");
int n, m;
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 = 1, r = 1e18;
while(l < r)
{
ll mid = (l + r + 1) / 2;
if(can(mid, n, m))
l = mid;
else
r = mid - 1;
}
cout << l;
return 0;
}
