#include<bits/stdc++.h>
#define newl '\n'
const int N = 3e5 + 10;
const int V = 1e9 + 10;
const long long INF = 1e18;
const long long M = 1e9 + 7;
long long a[N + 1],b[N + 1],n,m;
void readData(){
std::cin >> n >> m;
for(int i = 1;i <= n;++i){
std::cin >> a[i];
}
for(int i = 1;i <= n;++i){
std::cin >> b[i];
a[i] = std::max(a[i],b[i]);
}
}
bool f(long long mid){
long long s = 0;
for(int i = 1;i <= n;++i){
long long t = std::min(m,((mid - 1)) / a[i] + 1);
s += t;
if(a[i] * t < mid){
s += ((mid - a[i] * t - 1) / b[i] + 1);
}
if(s > m * n){
return false;
}
}
return true;
}
long long solve(){
long long lo = 0,hi = (long long)N * V,ans = 0;
while(lo <= hi){
long long mid = (lo + hi) / 2;
if(f(mid)){
ans = mid;
lo = mid + 1;
}else{
hi = mid - 1;
}
}
return ans;
}
int main(){
std::ios_base::sync_with_stdio(false);
std::cin.tie(nullptr);std::cout.tie(nullptr);
readData();
std::cout << solve();
return 0;
}
