#include <bits/stdc++.h>

#define int int64_t

const int kMaxN = 3e5 + 5;

int n, m;
int a[kMaxN], b[kMaxN];

bool check(int k) {
  int cnt = 0;
  for (int i = 1; i <= n; ++i) {
    int c = std::min(m, (k + a[i] - 1) / a[i]);
    cnt += std::min((k + b[i] - 1) / b[i], c + std::max<int>(0, (k - a[i] * c + b[i] - 1) / b[i]));
  }
  return cnt <= n * m;
}

void dickdreamer() {
  std::cin >> n >> m;
  for (int i = 1; i <= n; ++i) std::cin >> a[i];
  for (int i = 1; i <= n; ++i) std::cin >> b[i];
  int L = -1, R = 1e18, res = 1e18;
  while (L + 1 < R) {
    int mid = (L + R) >> 1;
    if (check(mid)) L = res = mid;
    else R = mid;
  }
  std::cout << res << '\n';
}

int32_t main() {
#ifdef ORZXKR
  freopen("in.txt", "r", stdin);
  freopen("out.txt", "w", stdout);
#endif
  std::ios::sync_with_stdio(0), std::cin.tie(0), std::cout.tie(0);
  int T = 1;
  // std::cin >> T;
  while (T--) dickdreamer();
  // std::cerr << 1.0 * clock() / CLOCKS_PER_SEC << "s\n";
  return 0;
}