#include <bits/stdc++.h>
using namespace std;
long long power(long long x, long long y, long long p) {
  long long res = 1;
  x = x % p;
  while (y > 0) {
    if (y & 1) res = (res * x) % p;
    y = y >> 1;
    x = (x * x) % p;
  }
  return res;
}
long long modInverse(long long n) {
  long long p = 1000000007;
  return power(n, p - 2, p);
}
long long min(long long a, long long b, long long c) {
  return min(a, min(b, c));
}
long long max(long long a, long long b, long long c) {
  return max(a, max(b, c));
}
inline void swap(long long &a, long long &b) {
  long long t = a;
  a = b;
  b = t;
}
bool cmp(pair<long long, long long> &a, pair<long long, long long> &b) {
  if (b.second < a.second) return true;
  if (b.second > a.second) return false;
  return a.first > b.first;
}
pair<long long, long long> read() {
  long long a, b;
  cin >> a >> b;
  return make_pair(a, b);
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long n, k;
  cin >> n >> k;
  vector<pair<long long, long long>> A;
  for (long long i = 0; i < n; i++) {
    A.push_back(read());
  }
  sort(A.begin(), A.end(), cmp);
  priority_queue<long long, vector<long long>, greater<long long>> q;
  long long sum = 0;
  long long ans = 0;
  for (long long i = 0; i < n; i++) {
    sum += A[i].first;
    long long Min = A[i].second;
    q.push(A[i].first);
    if (q.size() > k) {
      sum -= q.top();
      q.pop();
    }
    ans = max(ans, 1ll * sum * Min);
  }
  cout << ans;
  return 0;
}
