#include <bits/stdc++.h>
typedef long long ll;
using namespace std;
#define FOR(i, x, y) for(ll i=x; i<y; i++)
#define FORNEG(i, x, y) for(ll i=x; i>y; i--)
#define fast() ios_base::sync_with_stdio(false);cin.tie(NULL)
ll n,m;
vector<ll> lis;
vector<ll> lis2;
bool check(ll a){
ll cnt = 0;
FOR(i,0,n){
if (lis[i] > lis2[i]){
if (a>lis[i]*m){
cnt += ceil((a-(lis[i]*m))/lis2[i]) + m + !!((a-(lis[i]*m))%lis2[i]);
}else{
cnt += ceil(a/lis[i]) + !!(a%lis[i]);
}
}else{
cnt += ceil(a/lis2[i]) + !!(a%lis2[i]);
}
if (cnt > n*m){
return false;
}
}
if (cnt <= n*m){
return true;
}return false;
}
int main(){
cin >> n >> m;
FOR(i,0,n){
ll temp; cin >> temp;
lis.push_back(temp);
}
FOR(i,0,n){
ll temp; cin >> temp;
lis2.push_back(temp);
}
ll lo = 0;
ll hi = 1000000000000000000;
while (lo<hi){
ll mid = (lo+hi+1)/2;
if (check(mid)){
lo = mid;
}else{
hi = mid - 1;
}
}
cout << lo;
}
