#include <bits/stdc++.h>
#define pb push_back
#define fst first
#define snd second
#define fore(i,a,b) for(int i=a,pao=b;i<pao;++i)
#define SZ(x) ((int)x.size())
#define ALL(x) x.begin(),x.end()
#define mset(a,v) memset((a),(v),sizeof(a))
#define FIN ios::sync_with_stdio(0);cin.tie(0);cout.tie(0)
using namespace std;
typedef long long ll;
int main(){
int n,k;cin>>n>>k;
vector<int>a(n),b(n);
vector<pair<int,int>>v;
fore(i,0,n){
cin>>a[i]>>b[i];
v.pb({a[i],b[i]});
}
sort(ALL(v));
double sum=0;
fore(i,0,k)sum+=v[i].fst;
cout<<fixed<<setprecision(20)<<sum<<endl;
}
