#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define mp make_pair
#define f1 first
#define s2 second
#define fastio ios :: sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define debug(x) cerr << "[" << #x << "]: " << x << "\n";
using ll = long long;
using ld = long double;
using ii = pair<int, int>;
using pl = pair<ll, ll>;
constexpr ld PI = 4*atan((ld)1);
constexpr int MAX = 3e5 + 69;
ll n, m;
ll a[MAX], b[MAX];
#define ll __int128_t
inline bool check(ll val)
{
ll sum = 0;
for (int i = 0; i < n; ++i)
{
ll cur = val;
if (a[i] > b[i])
{
ll attend = min((__int128_t)m, (cur + a[i] - 1) / a[i]);
sum += attend;
cur -= attend * a[i];
}
if (cur > 0) sum += (cur + b[i] - 1) / b[i];
}
return sum <= n*m;
}
#undef ll
int main()
{
fastio;
cin >> n >> m;
for (int i = 0; i < n; ++i)
cin >> a[i];
for (int i = 0; i < n; ++i)
cin >> b[i];
ll l = 1, r = 1e18;
while (l < r)
{
ll mid = (l + r + 1) >> 1;
if (check(mid)) l = mid;
else r = mid-1;
}
cout << l << '\n';
return 0;
}
