#include "bits/stdc++.h"
using namespace std;
#define int long long
/*
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*/
template<class T> bool setmax(T& a, const T& b){
return b < a ? a = b, 1 : 0;
}
template<class T> bool setmin(T& a, const T& b){
return a < b ? a = b, 1 : 0;
}
signed main()
{
int n , m;
cin >> n >> m;
vector<int> a(n) , b(n);
for (auto & x : a)
cin >> x;
for (auto & x : b)
cin >> x;
auto check = [&](int p){
int t = 0;
for (int i = 0; i < n; i++){
if (b[i] > a[i])
t += (p + b[i] - 1) / b[i];
else {
if (a[i] * 1ll * m >= p)
t += (p + a[i] - 1) / a[i];
else
t += m + (p - a[i] * 1ll * m + b[i] - 1) / b[i];
}
if (t > n * 1ll * m)
return false;
}
if (t <= n * 1ll * m)
return true;
return false;
};
int l = 0 , r = 1e18 + 7;
while(l < r){
int m = (l + r + 1) >> 1;
if (check(m))
l = m;
else r = m - 1;
}
cout << l << "\n";
return 0;
}
