#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  vector<pair<long long, long long> > v(n);
  for (int i = 0; i < n; i++) cin >> v[i].second >> v[i].first;
  sort(v.rbegin(), v.rend());
  long long ans = 0, sum = 0;
  priority_queue<long long> pq;
  for (int i = 0; i < n; i++) {
    pq.push(v[i].second);
    sum += v[i].second;
    if (pq.size() > k) {
      sum -= pq.top();
      pq.pop();
    }
    ans = max(ans, v[i].first * sum);
  }
  cout << ans << endl;
  return 0;
}
