#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define int long long
#define FOR(i, a, b) for ( int i = a ; i <= b ; i++ )
#define FORD(i, a, b) for (int i = b; i >= a; i --)
#define REP(i, n) for (int i = 0; i < n; ++i)
#define REPD(i, n) for (int i = n - 1; i >= 0; --i)
#define pii pair<int , int>
#define fi first
#define se second
#define Lg(x) 31 - __builtin_clz(x)
#define MASK(i) (1LL << (i))
#define BIT(x, i) (((x) >> (i)) & 1)
constexpr ll LINF = (1ll << 60);
constexpr int INF = (1ll << 30);
constexpr int MAX = 1e6 + 5;
constexpr int Mod = 1e9 + 7;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
void setupIO(){
#define name "Whisper"
//Phu Trong from Nguyen Tat Thanh High School for gifted student
srand(time(NULL));
cin.tie(nullptr)->sync_with_stdio(false); cout.tie(nullptr);
//freopen(name".inp", "r", stdin);
//freopen(name".out", "w", stdout);
cout << fixed << setprecision(10);
}
template <class X, class Y>
bool minimize(X &x, const Y &y){
X eps = 1e-9;
if (x > y + eps) {x = y; return 1;}
return 0;
}
template <class X, class Y>
bool maximize(X &x, const Y &y){
X eps = 1e-9;
if (x + eps < y) {x = y; return 1;}
return 0;
}
int numHouse, numFamily;
struct Data{
int width, height;
bool operator < (const Data& oth) const{
return height < oth.height;
}
} a[MAX];
void Whisper(){
cin >> numHouse >> numFamily;
for (int i = 1; i <= numHouse; ++i){
cin >> a[i].width >> a[i].height;
}
sort(a + 1, a + numHouse + 1);
priority_queue<int> q;
int tot = 0;
int ans = LINF;
for (int i = 1; i <= numHouse; ++i){
while ((int)q.size() >= numFamily) tot -= q.top(), q.pop();
q.push(a[i].width); tot += a[i].width;
if ((int)q.size() == numFamily)
minimize(ans, tot * a[i].height);
}
cout << ans;
}
signed main(){
setupIO();
int Test = 1;
// cin >> Test;
for ( int i = 1 ; i <= Test ; i++ ){
Whisper();
if (i < Test) cout << '\n';
}
}
