#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
using namespace std;
const int mod = 1e9 + 7;
clock_t t1, t2;
long long binPow(long long a, long long b) {
  long long x = 1, y = a;
  while (b) {
    if (b & 1) x = (x * y) % mod;
    y = (y * y) % mod;
    b >>= 1;
  }
  return x % mod;
}
long long inverserEuler(int n) { return binPow(n, mod - 2); }
long long C(int k, int n) {
  vector<long long> f(n + 1, 1);
  for (int i = 2; i <= n; i++) f[i] = (f[i - 1] * i) % mod;
  return (f[n] * ((inverserEuler(f[k]) * inverserEuler(f[n - k])) % mod) %
          mod) %
         mod;
}
const int maxn = 3e5 + 5;
pair<int, int> a[maxn];
long long n, k;
void trunghieu() {
  cin >> n >> k;
  int i, j;
  for (i = 0; i < n; ++i) cin >> a[i].second >> a[i].first;
  sort(a, a + n);
  set<pair<int, int>> s;
  long long ans = 0;
  long long sum = 0;
  for (i = n - 1; i >= 0; i--) {
    s.insert({a[i].second, i});
    sum += a[i].second;
    while (s.size() > k) {
      auto u = s.begin();
      sum -= u->first;
      s.erase(u);
    }
    ans = max(ans, sum * a[i].first);
  }
  cout << ans;
}
int main() {
  cerr << "Program is running" << endl;
  t1 = clock();
  ios::sync_with_stdio(false);
  cin.tie();
  trunghieu();
  t2 = clock();
  float diff((float)t2 - (float)t1);
  float seconds = diff / CLOCKS_PER_SEC;
  cerr << "\nRunning in " << seconds << " seconds" << endl;
  return 0;
}
