#include <bits/stdc++.h>
using namespace std;
int main() {
  priority_queue<pair<int, int> > pq;
  vector<pair<int, pair<int, int> > > ms, ori;
  int n, m;
  cin >> n >> m;
  for (int i = 0; i < n; i++) {
    int a, b;
    cin >> a >> b;
    pair<int, pair<int, int> > t = make_pair(b, make_pair(a, i));
    ms.push_back(t);
  }
  sort(ms.begin(), ms.end());
  for (int i = 0; i < ms.size(); i++) {
    pq.push(make_pair(ms[i].second.first, i));
  }
  set<int> st;
  long long sigL = 0;
  for (int i = 0; i < m; i++) {
    int val = pq.top().first;
    int id = pq.top().second;
    pq.pop();
    sigL += val;
    st.insert(id);
  }
  long long ans = ms[0].first * sigL;
  for (int i = 1; i < n; i++) {
    if (st.find(i - 1) != st.end()) {
      while (!pq.empty() && pq.top().second < i) pq.pop();
      if (pq.empty()) break;
      pair<int, int> t = pq.top();
      pq.pop();
      sigL -= ms[i - 1].second.first;
      st.erase(i - 1);
      sigL += t.first;
      st.insert(t.second);
    }
    ans = max(ans, sigL * ms[i].first);
  }
  for (int i = n - m; i < n - 1; i++) {
    sigL -= ms[i].second.first;
    ans = max(ans, sigL * ms[i + 1].first);
  }
  cout << ans << endl;
  return 0;
}
