#include <bits/stdc++.h>
using namespace std;
const int inf = numeric_limits<int>::max();
const long long int linf = numeric_limits<long long int>::max();
const int mod = (int)1e9 + 7;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int k, n;
  cin >> n >> k;
  int a, b;
  vector<pair<long long int, long long int>> v;
  for (int i = 0; i < (n); ++i) {
    cin >> a >> b;
    v.push_back(make_pair(b, a));
  }
  sort(v.begin(), v.end());
  long long int res = 0;
  long long int sum = 0;
  set<pair<int, int>> s;
  for (int i = n - 1; i > -1; i--) {
    s.insert(make_pair(v[i].second, i));
    sum += v[i].second;
    while (s.size() > k) {
      auto it = s.begin();
      sum -= it->first;
      s.erase(it);
    }
    res = max(res, (long long int)sum * v[i].first);
  }
  cout << res << "\n";
  return 0;
}
