#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <set>
#include <stack>
#include <queue>
using namespace std;
using pii = pair<int, int>;
using vpii = vector<pii>;
using ll = long long;
using vll = vector<ll>;
using vvll = vector<vll>;
using vi = vector<int>;
using vvi = vector<vi>;
#define sz(x) int(x.size())
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
ll N, M;
cin >> N >> M;
vll A(1+N), B(1+N);
for(int i = 1; i <= N; i++) cin >> A[i];
for(int i = 1; i <= N; i++)
{
cin >> B[i];
A[i] = max(A[i], B[i]);
}
ll lo = 0, hi = 1'000'000'000'000'000'000LL;
while(lo != hi)
{
ll target = (lo+hi)/2 + 1;
vll remaining_goal(1+N);
ll remaining_slots = 0;
for(int i = 1; i <= N; i++)
{
ll classes = min(M, (target / A[i]) + bool(target % A[i]));
remaining_goal[i] = max(0LL, target - classes * A[i]);
remaining_slots += M - classes;
}
for(int i = 1; i <= N; i++)
{
ll curr = (remaining_goal[i] / B[i]) + bool(remaining_goal[i] % B[i]);
remaining_slots -= curr;
if(remaining_slots < 0)
{
break;
}
}
// cerr << target << " : " << remaining_slots << '\n';
if(remaining_slots >= 0)
lo = target;
else
hi = target - 1;
}
cout << lo << '\n';
}
