#include <bits/stdc++.h>
using namespace std;
inline long long read() {
  long long x = 0, f = 0;
  char ch = getchar();
  while (ch > '9' || ch < '0') f |= ch == '-', ch = getchar();
  while (ch >= '0' && ch <= '9')
    x = (x << 3) + (x << 1) + (ch ^ 48), ch = getchar();
  return f ? -x : x;
}
struct point {
  long long a, b;
  bool operator<(const point& t) const { return b > t.b; }
} p[300005];
long long max(long long a, long long b) { return a > b ? a : b; }
int main() {
  int n = read(), k = read();
  for (int i = 0; i < n; ++i) p[i] = {read(), read()};
  sort(p, p + n);
  priority_queue<long long, vector<long long>, greater<long long>> q;
  long long sum = 0, ans = 0;
  for (int i = 0; i < n; ++i) {
    q.push(p[i].a);
    sum += p[i].a;
    if (q.size() > k) sum -= q.top(), q.pop();
    ans = max(ans, sum * p[i].b);
  }
  printf("%I64d\n", ans);
  return 0;
}
