// 還要更強
#include<iostream>
#include<queue>
#include<set>
#include<map>
#include<iomanip>
#include<math.h>
#include<cstring>
#include<stack>
#include<string.h>
#include<random>
#include<algorithm>
#include<vector>
#include <cassert>
#define ll long long
#define FOR(i,a,b) for(ll i=a;i<b;i++)
#define REP(i,a,b) for(int i=a;i>=b;i--)
#define INF (ll)(1e9)
#define pb push_back
#define F first
#define S second
using namespace std;
ll n,m;
ll aaa[300005],bbb[300005];
signed main(){
cin>>n>>m;
FOR(i,1,n+1) cin>>aaa[i];
FOR(i,1,n+1) cin>>bbb[i];
ll l=0,r=(1e18+1),ans=0;
while(l<r){
ll mid=(l+r)/2,tot=0;
FOR(i,1,n+1){
aaa[i]=max(aaa[i],bbb[i]);
if(aaa[i]*m<mid){
ll tmp=mid-aaa[i]*m;
tot+=abs((tmp-1)/bbb[i]+1)+INF;
}
else{
//ll tmp=(mid-1)/aaa[i]+1;
//tot-=m-tmp;
}
}
//cout<<"mid="<<mid<<" tot="<<tot<<endl;
if(tot<=0){
ans=max(ans,mid);
l=mid+1;
}
else{
r=mid;
}
}
cout<<ans<<endl;
}
