#include <bits/stdc++.h>
using namespace std;
const int MAX = 1e5 + 55;
const int INF = 1e9 + 77;
const int MOD = 1e9 + 7;
const double PI = acos(-1.0);
vector<int> months = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
long long power(long long x, long long y) {
  if (y == 0) return 1;
  long long s = power(x, y / 2);
  s *= s;
  if (y % 2 == 1) s *= x;
  return s;
}
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
long long fac(long long x) {
  long long p = 1;
  for (long long i = 2; i <= x; i++) {
    p *= i;
  }
  return p;
}
long long n, k;
pair<long long, long long> x[3 * MAX];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> k;
  for (int i = 0; i < n; ++i) {
    cin >> x[i].second >> x[i].first;
  }
  sort(x, x + n, greater<pair<long long, long long>>());
  priority_queue<long long, vector<long long>, greater<long long>> pq;
  pq.push(x[0].second);
  long long sum = x[0].second;
  long long ans = x[0].first * x[0].second;
  for (int i = 1; i < k; i++) {
    sum += x[i].second;
    ans = max(ans, sum * x[i].first);
    pq.push(x[i].second);
  }
  for (int i = k; i < n; i++) {
    long long p = pq.top();
    sum -= pq.top();
    pq.pop();
    sum += x[i].second;
    ans = max(ans, x[i].first * sum);
    pq.push(max(p, x[i].second));
    sum -= x[i].second;
    sum += max(p, x[i].second);
  }
  cout << ans;
  return 0;
}
