#include <bits/stdc++.h>
using namespace std;
#define int long long
const int MAXN = 3*1e5+5;
int a[MAXN], b[MAXN], n,m;
int teto(int x, int y){
return (x + (y-1))/y;
}
bool test(int h){
int tempo = 0;
for(int i = 1; i <= n; i++){
if(max(a[i],b[i])*m >= h){
tempo += teto(h, max(a[i],b[i]) );
}
else{
tempo += m + teto( h - max(a[i],b[i])*m, b[i]);
}
}
return tempo <= n*m;
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cin>>n>>m;
for(int i = 1; i <= n; i++)cin>>a[i];
for(int i = 1; i <= n; i++)cin>>b[i];
int l = 0, r = 1e18;
while(l != r){
int mid = (l+r+1)/2;
if(test(mid)) l = mid;
else r = mid-1;
}
cout<<l<<"\n";
}
