//#pragma GCC optimize("Ofast", "unroll-loops")
//#pragma GCC target("sse", "sse2", "sse3", "ssse3", "sse4")
#include <bits/stdc++.h>
#define all(a) a.begin(),a.end()
#define len(a) (int)(a.size())
#define mp make_pair
#define pb push_back
#define fir first
#define sec second
#define fi first
#define se second
using namespace std;
typedef pair<int, int> pii;
typedef long long ll;
typedef long double ld;
template<typename T>
bool umin(T &a, T b) {
if (b < a) {
a = b;
return true;
}
return false;
}
template<typename T>
bool umax(T &a, T b) {
if (a < b) {
a = b;
return true;
}
return false;
}
#ifdef KIVI
#define DEBUG for (bool _FLAG = true; _FLAG; _FLAG = false)
#define LOG(...) print(#__VA_ARGS__" ::", __VA_ARGS__) << endl
template<class ...Ts>
auto &print(Ts ...ts) { return ((cerr << ts << " "), ...); }
#else
#define DEBUG while (false)
#define LOG(...)
#endif
mt19937 rng(4242);
const int max_n = -1, inf = 1000111222;
void solve() {
int n, m;
cin >> n >> m;
vector<ll> a(n), b(n);
for(auto& x : a) cin >> x;
for(auto& x : b) cin >> x;
ll l = 0, r = 1000111222000111222;
while(l < r) {
ll x = (l + r + 1) / 2;
ll need = 0;
for(int i = 0; i < n; i++) {
ll take_a = min((ll)m, (x + a[i] - 1) / a[i]);
if(a[i] <= b[i]) take_a = 0;
ll take_b = (max(0ll, x - take_a * a[i]) + b[i] - 1) / b[i];
need += take_a + take_b;
}
if(need <= n * 1ll * m) l = x;
else r = x - 1;
}
cout << l << '\n';
}
int main() {
// freopen("input.txt", "r", stdin);
// freopen("output.txt", "w", stdout);
ios_base::sync_with_stdio(0);
cin.tie(0);
int t = 1;
// cin >> t;
while (t--) solve();
}
/*
KIVI
*/
