#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
const int MAXN = 3e5 + 7;
pair<ll, ll> courses[MAXN];
ll n, m;
bool check(ll mid){
ll tickets = n*m;
for(int i = 1; i <= n; i++){
ld tmp = mid;
tickets -= min(m, (ll)ceil(tmp/courses[i].first));
tmp -= min(m, (ll)ceil(tmp/courses[i].first)) * courses[i].first;
if(tmp > 0)
tickets -= (ll)ceil(tmp/courses[i].second);
if(tickets < 0) return false;
}
return true;
}
ll BS(){
ll l = 1, r = (m * 1e9), mid;
while(l < r){
mid = (l+r)/2;
if(check(mid)) l = mid + 1;
else r = mid;
}
if(!check(l)) l--;
return l;
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> n >> m;
for(int i = 1; i <= n; i++)
cin >> courses[i].first;
for(int i = 1; i <= n; i++){
cin >> courses[i].second;
courses[i].first = max(courses[i].first, courses[i].second);
}
cout << BS() << "\n";
}
