// 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>
#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)(2e18)
#define pb push_back
#define F first
#define S second
using namespace std;
ll n,m;
ll a[300005],b[300005];
bool check(ll x){
ll res=0;
FOR(i,0,n){
ll tmp=x;
if((a[i]*m)<=tmp){
tmp-=(a[i]*m);
if(tmp){
tmp=((tmp-1)/b[i])+1;
}
if(tmp) res-=tmp;
}
else{
//ll used=(tmp-1)/a[i]+1;
//res+=m-used;
}
}
if(res>=0) return 1;
return 0;
}
signed main(){
cin>>n>>m;
FOR(i,0,n) cin>>a[i];
FOR(i,0,n){
cin>>b[i]; a[i]=max(a[i],b[i]);
}
ll l=0,r=(2e18+1),ans=0;
while(l<r){
ll mid=(l+r)>>1;
if(check(mid)){
ans=max(ans,mid); l=mid+1;
}
else{
r=mid;
}
}
cout<<ans<<endl;
}
