#include <bits/stdc++.h>
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define int long long
#define yes() cout << "YES\n"
#define no() cout << "NO\n"
using namespace std;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
const int inf = 1e18;
const int mod = 1e9 + 7;
const int maxn = 1e7 + 5e6;
const int mod1 = 998244353;
const int mod2 = 1e18 + 1;
const int mod3 = 1e9 + 9;
const int mod4 = 333333333;
const int mod5 = 200000;
const int mod6 = 10007;
const int k = 300;
const int w = 1e5;
const ld EPS = 1e-8;
int LOG = 30;
void solve() {
int n, m;
cin >> n >> m;
vector<int> a(n), b(n);
for (int& i : a) cin >> i;
for (int& i : b) cin >> i;
int l = 0;
int r = 1e15;
auto check = [&](int x) {
vector<int> prime(n), cnt(n, m);
for (int i = 0; i < n; i++) {
int y = max(a[i], b[i]);
int c = min(m, (x + y - 1) / y);
cnt[i] -= c;
prime[i] += c * y;
}
vector<int> need;
vector<int> free;
for (int i = 0; i < n; i++) if (prime[i] < x) need.push_back(i);
for (int i = 0; i < n; i++) if (cnt[i] > 0) free.push_back(cnt[i]);
int res = need.size();
for (int& i : need) {
int y = b[i];
int must = x - prime[i];
int gol = (must + y - 1) / y;
while (free.size() && gol > 0) {
int much = free.back();
free.pop_back();
if (gol < much) {
much -= gol;
gol = 0;
} else {
gol -= much;
much = 0;
}
if (much > 0) free.push_back(much);
}
if (gol == 0) res--;
else break;
}
return !res;
};
while (r - l > 1) {
int mid = (l + r) / 2;
if (check(mid)) l = mid;
else r = mid;
}
cout << l << "\n";
}
signed main() {
// cout.precision(16);
ios::sync_with_stdio(false);
cin.tie(nullptr);
int t = 1;
// cin >> t;
while (t--) {
solve();
}
}
