#include<bits/stdc++.h>
#include<random>
using namespace std;
template<typename T> void _do(T x){cerr<<x<<"\n";}
template<typename T,typename ...U> void _do(T x,U ...y){cerr<<x<<", ";_do(y...);}
#define dbg(...) cerr<<#__VA_ARGS__<<" = ";_do(__VA_ARGS__);
#define float double
#define ss(n) fixed<<setprecision(n)
#define ll long long
#define int ll
#define IO ios::sync_with_stdio(false);cin.tie(0);
#define ld long double
#define pb push_back
#define pii pair<int,int>
#define rep(i,a) for(int i=1;i<=a;i++)
#define rep0(i,a) for(int i=0;i<a;i++)
#define F first
#define S second
#define uni(a) a.resize(distance(a.begin(),unique(a.begin(),a.end())));
#define unisort(a) sort(a.begin(),a.end()); uni(a);
ll gcd(ll a,ll b){if(b==0) return a; return gcd(b,a%b);}
const ld pi=3.14159265358979323846264338327950288419716939931;
const int lar=3e18;
const int mol1=1e9+7;
const int mol2=998244353;
const int MX=3e5;
void solve()
{
int n,m;
cin>>n>>m;
int a[n+1],b[n+1];
rep(i,n) cin>>a[i];
rep(i,n) cin>>b[i];
if(n*m<=300000){
priority_queue<pair<int,int>,vector<pair<int,int>>,greater<>>now;
rep(i,n) now.push({0,i});int vis[n+1]={0};rep(j,n) vis[j]=0;
rep(i,m*n){
pair<int,int>kk=now.top();
if(vis[kk.S]<m){
kk.F+=max(a[kk.S],b[kk.S]);
vis[kk.S]++;
}
else {
kk.F+=b[kk.S];
}
now.pop();
now.push(kk);
}
cout<<now.top().F<<"\n";}
else{
int l=m,r=1e18;
int ans=0;
while(l<r){
int tt=0;
int mid=l+r+1>>1;
rep(i,n){
if(a[i]>b[i]){
if(a[i]*m>=mid){
tt+=(mid/a[i])+(mid%a[i]?1:0);
}
else{
tt+=m;
int g1=mid-a[i]*m;
tt+=(g1/b[i])+(g1%b[i]?1:0);
}
}
else{
tt+=(mid/b[i])+(mid%b[i]?1:0);
}
}
if(tt<=n*m) {
ans=max(ans,mid);
l=mid;
}
else{
r=mid-1;
}
}
cout<<ans<<"\n";
}
}
signed main()
{
IO
solve();
return 0;
}
