#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, m, ans = 0, res = 0;
  cin >> n >> m;
  vector<pair<long long, long long>> v;
  set<pair<long long, long long>> s;
  for (int i = 0; i < n; i++) {
    long long p, q;
    cin >> p >> q;
    v.push_back(make_pair(q, p));
  }
  sort(v.begin(), v.end(), greater<pair<long long, long long>>());
  for (int i = 0; i < n; i++) {
    res += v[i].second;
    s.insert(make_pair(v[i].second, i));
    while (s.size() > m) {
      auto it = s.begin();
      res -= it->first;
      s.erase(it);
    }
    ans = max(ans, res * v[i].first);
  }
  cout << ans << endl;
  return 0;
}
