#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define ll long long
#define ld long double
#define ull unsigned long long
#define ff first
#define ss second
#define pii pair<int,int>
#define pll pair<long long, long long>
#define vi vector<int>
#define vl vector<long long>
#define pb push_back
#define rep(i, b) for(int i = 0; i < (b); ++i)
#define rep2(i,a,b) for(int i = a; i <= (b); ++i)
#define rep3(i,a,b,c) for(int i = a; i <= (b); i+=c)
#define count_bits(x) __builtin_popcountll((x))
#define all(x) (x).begin(),(x).end()
#define siz(x) (int)(x).size()
#define forall(it,x) for(auto& it:(x))
using namespace __gnu_pbds;
using namespace std;
typedef tree<int, null_type, less<int>, rb_tree_tag,tree_order_statistics_node_update> ordered_set;
//mt19937 mt;void random_start(){mt.seed(chrono::time_point_cast<chrono::milliseconds>(chrono::high_resolution_clock::now()).time_since_epoch().count());}
//ll los(ll a, ll b) {return a + (mt() % (b-a+1));}
const int INF = 1e9+50;
const ll INF_L = 1e18+40;
const ll MOD = 1e9+7;
pll arr[300'001];
int n;
ll m;
bool check(ll mid)
{
ll opers = 0;
rep(i,n)
{
ll o = (mid + arr[i].ff-1)/arr[i].ff;
if(o <= m)
{
opers += m-o;
}
else
{
ll zost = (mid - m*arr[i].ff+arr[i].ss-1)/arr[i].ss;
if(zost <= opers)
{
opers -= zost;
}
else return 0;
}
}
return 1;
}
int main()
{
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
//random_start();
cin >> n >> m;
rep(i,n) cin >> arr[i].ff;
rep(i,n) cin >> arr[i].ss;
rep(i,n) arr[i].ff = max(arr[i].ff,arr[i].ss);
sort(arr,arr+n);
reverse(arr,arr+n);
ll l = 0;
ll r = 1e18;
ll ans = 0;
while(l <= r)
{
ll mid = (l+r)/2;
if(check(mid))
{
ans = mid;
l = mid+1;
}
else
{
r = mid-1;
}
}
cout << ans << "\n";
}
