// #include <bits/stdc++.h>
#include <iostream>
#include <cmath>
#include <algorithm>
#include <map>
#include <unordered_map>
#include <vector>
#include <iomanip>
#include <string>
#include <queue>
#include <set>
#include <deque>
using namespace std;
#define int long long
#define endl "\n"
#define fi first
#define se second
const int M=1e9+7;
const int inf = 1e14;
const int LOG=18;
const int N=5e1+5;
int n , m , c , w , k , t=1 , q=1 , x , y , z , l , r;
void solve(){
cin >> n >> m;
vector<int>a(n),b(n);
for (int i=0;i<n;i++){
cin >> a[i];
}
for (int i=0;i<n;i++){
cin >> b[i];
}
int l=0;
int h=1e18;
int mi=(l+h+1)/2;
while (l<h){
int op=0;
for (int i=0;i<n;i++){
int c=max(a[i],b[i]);
int u=(mi+c-1)/(c);
x=mi-min(u,m)*c;
op+=min(u,m);
u-=min(u,m);
if (u){
u=(x+b[i]-1)/b[i];
op+=u;
}
}
if (op<=n*m){
l=mi;
}
else{
h=mi-1;
}
mi=(l+h+1)/2;
}
cout << mi << endl;
}
signed main()
{
ios::sync_with_stdio(0);//DO NOT USE IN INTERACTIVE
cin.tie(0), cout.tie(0);//DO NOT USE IN INTERACTIVE
cout << fixed << setprecision(9);
srand(time(0));
// int t=1;
// cin >> t;
for (int _=1;_<=t;_++){
solve();
q++;
}
}
