#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define all(x) x.begin(),x.end()
#define int long long
#define ar array
template<class T>bool umax(T &a,T b){if(a<b){a=b;return true;}return false;}
template<class T>bool umin(T &a,T b){if(b<a){a=b;return true;}return false;}
namespace FAST {
template<typename T>
istream &operator>>(istream &cin, vector<T> &a) {
for (T &i: a) cin >> i;
return cin;
}
template<typename T>
ostream &operator<<(ostream &cout, vector<T> &a) {
for (T i: a) cout << i << ' ';
return cout;
}
}
using namespace FAST;
const int inf = 3e18 + 7;
const int mod = 1e9 + 7;
const int N = 6e5 + 5;
const int md = 998244353;
int n,m,k;
void solve(){
cin >> n >> m;
vector<int>a(n), b(n);
cin >> a >> b;
int l = 1, r = 1e18, ans = -1;
while(l <= r){
int mid = (l + r) / 2;
//~ mid = 18;
int cnt = 0;
for(int i = 0;i<n;i++){
int mx = max(a[i], b[i]);
if(mx * m >= mid){
int cont = (mid + mx - 1) / mx;
cnt += m - cont;
}
}
for(int i = 0;i<n;i++){
int mx = max(a[i], b[i]);
if(mx * m >= mid)continue;
int u = mid;
u -= m * mx;
int cont = (u + b[i] - 1) / b[i];
cnt -= cont;
}
if(cnt >= 0){
l = mid + 1;
ans = mid;
}
else r = mid - 1;
//~ return;
}
cout << ans << "\n";
}
/*
*/
signed main()
{
// freopen("seq.in", "r", stdin);
// freopen("seq.out", "w", stdout);
ios_base::sync_with_stdio(0);cin.tie(NULL);cout.tie(NULL);
int tt=1;//cin >> tt;
while(tt--)solve();
}
