#include<algorithm>
#include<iostream>
#include<queue>
#include<map>
#define ll long long
using namespace std;
ll prefsum[1000005];
vector<pair<int,int> > bprints_h;
vector<pair<int,int> > bprints_w;
map<pair<int,int>,int> M;
map<int,pair<int,int> > used;
int main() {
cin.sync_with_stdio(0); cin.tie(0);
cin.exceptions(cin.failbit);
int n, k, w, h;
cin >> n >> k;
for(int i=0;i<n;i++) {
cin >> w >> h;
bprints_h.push_back(make_pair(h,w));
bprints_w.push_back(make_pair(w,h));
}
sort(bprints_h.begin(),bprints_h.end(),greater<pair<int,int> >());
sort(bprints_w.begin(),bprints_w.end());
M.insert(make_pair(make_pair(bprints_w[0].first, bprints_w[0].second),0));
prefsum[0] = bprints_w[0].first;
for(int i=1;i<n;i++) {
M.insert(make_pair(make_pair(bprints_w[i].first, bprints_w[i].second),i));
prefsum[i] = prefsum[i-1] + bprints_w[i].first;
}
int index, i=0;
ll diff = 0, sum = 0, sol = 1000000000000000000;
while(n>=k) {
index = M.find(make_pair(bprints_h[i].second,bprints_h[i].first))->second;
sum = prefsum[k-1];
sum -= diff;
sol = min(sol, bprints_h[i].first*sum);
if(index>k) used.insert(make_pair(index,make_pair(bprints_h[i].second,bprints_h[i].first)));
if(index<=k) {
k++; diff += bprints_h[i].second;
while(used.find(k)!=used.end()) {
diff += (used.find(k)->second.first);
k++;
}
}
i++;
}
cout << sol;
return 0;
}
