#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define fst first
#define snd second
#define pb push_back
#define forn(i,a,b) for(int i = a; i < b; i++)
#define ALL(x) x.begin(),x.end()
#define SZ(x) (int)x.size()
#define mset(a,v) memset((a),(v),sizeof(a))
#define FIN ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define dbg(v) cout<<"Line("<<__LINE__<<"): "<<#v<<" = "<<v<<'\n';
#define pi pair<int,int>
#define pll pair<ll,ll>
typedef long long ll;
using namespace std;
using namespace __gnu_pbds;
typedef tree<int,null_type,less<int>,rb_tree_tag,tree_order_statistics_node_update> indexed_set;
typedef tree<int,null_type,less_equal<int>,rb_tree_tag,tree_order_statistics_node_update> indexed_multiset;
int main(){
ll n,m; cin>>n>>m;
vector<ll> a(n); forn(i,0,n) cin>>a[i];
vector<ll> b(n); forn(i,0,n) cin>>b[i];
forn(i,0,n) a[i]=max(a[i],b[i]);
ll l = 0; ll r = 9000000000000000000+5;
while(l<=r){
ll mid=(l+r)/2;
//cout<<mid<<'\n';
ll libres = 0;
vector<ll> falta(n,mid);
forn(i,0,n){
if(m*a[i]<mid){
falta[i]=mid-(m*a[i]);
continue;
}else{
falta[i]=0;
}
ll nl = mid/a[i] + (mid%a[i]!=0?1:0);
libres+=max((ll)0,m-nl);
}
forn(i,0,n) if(falta[i]>0){
ll nec = falta[i]/b[i] + (falta[i]%b[i]!=0?1:0);
libres-=nec;
}
if(libres>=0) l=mid+1;
else r=mid-1;
}
cout<<r<<'\n';
return 0;
}
