#include <bits/stdc++.h>
using namespace std;
const int inf = 0x3f3f3f3f;
const int _inf = 0xc0c0c0c0;
const long long INF = 0x3f3f3f3f3f3f3f3f;
const long long _INF = 0xc0c0c0c0c0c0c0c0;
const long long mod = (int)1e9 + 7;
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
long long ksm(long long a, long long b, long long mod) {
  int ans = 1;
  while (b) {
    if (b & 1) ans = (ans * a) % mod;
    a = (a * a) % mod;
    b >>= 1;
  }
  return ans;
}
long long inv2(long long a, long long mod) { return ksm(a, mod - 2, mod); }
const int MAX_N = 300025;
int n, m;
long long minn[MAX_N], sum[MAX_N];
struct node {
  long long a, b;
  bool operator<(const node other) const {
    if (b == other.b) return a < other.a;
    return b < other.b;
  }
} arr[MAX_N];
priority_queue<pair<long long, long long> > q;
int main() {
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= n; ++i) scanf("%lld%lld", &arr[i].a, &arr[i].b);
  sort(arr + 1, arr + 1 + n);
  long long sum = 0, cnt = 0, ans = 0;
  for (int i = n; i >= 1; --i) {
    if (cnt < m) {
      sum += arr[i].a;
      q.push(pair<long long, long long>(-arr[i].a, arr[i].b));
      cnt++;
    } else {
      pair<long long, long long> top = q.top();
      if (arr[i].a > (-top.first)) {
        sum -= (-top.first);
        sum += arr[i].a;
        q.pop();
        q.push(pair<long long, long long>(-arr[i].a, arr[i].b));
      }
    }
    ans = max(ans, arr[i].b * sum);
  }
  printf("%lld\n", ans);
  return 0;
}
