#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

int main() {
    int N, K; cin >> N >> K;
    vector<pair<int,int> > v(N);
    for (int i = 0; i < N; ++i) {
        cin >> v[i].second >> v[i].first;
    }
    sort(v.begin(), v.end(), greater<pair<int,int> >());
    ll sum = 0;
    ll res = 0;
    multiset<int> s;
    for (int i = 0; i < N; ++i) {
        int b =t> s;
    for (int i = 0; i < N; ++i) {
        int b = v[i].first;
        if (i < K) {
            sum += v[i].second;
            s.insert(v[i].second);
        }
        else {
            if (v[i].second > *s.begin()) {
                sum -= *s.begin();
                sum += v[i].second;
                s.erase(s.begin());
                s.insert(v[i].second);
            }

        }

        res = max(res, sum*b);
    }
    cout << res;
}