#include <bits/stdc++.h>
using namespace std;
#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
#define F first
#define S second
#define pb push_back
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<ll> vi;
int main() {
cin.tie(0)->sync_with_stdio(0);
cin.exceptions(cin.failbit);
ll n,m;cin>>n>>m;
vi a(n);rep(i,0,n)cin>>a[i];
vi b(n);rep(i,0,n)cin>>b[i];
bool eq = true;
rep(i,0,n)if(a[i]!=b[i]) eq=false;
ll worst = 1e18;
if(m==1){
rep(i,0,n) worst=min(worst,m*max(a[i],b[i]));
}else if(eq){
ll lo = 0;
ll hi = 1e18;
while(lo+1<hi){
ll mid = lo+(hi-lo)/2;
ll cost = 0;
rep(i,0,n) cost += (mid+a[i]-1)/a[i];
if(cost<=n*m) lo=mid;
else hi=mid;
}
worst=lo;
}else if(n*m <= 300000){
// priority_queue<pair<ll,int>,vector<pair<ll,int>>,greater<pair<ll,int>>> pq;
// rep(i,0,n) pq.push({0,i});
// rep(_,0,n*m){
// auto p = pq.top(); pq.pop();
// pq.push({p.F + a[p.S], p.S});
// }
// worst = pq.top().F;
}
cout<<worst<<endl;
}
