#include <bits/stdc++.h>
using namespace std;
int const NMAX=505;
int n,k;
struct country{
int a,b;
bool votat;
}v[NMAX];
void read(){
cin>>n>>k;
int i;
for(i=1;i<=n;++i)
cin>>v[i].a>>v[i].b;
}
bool crt_a(country c1,country c2){
return c1.a<c2.a;
}
bool crt_b(country c1,country c2){
return c1.b<c2.b;
}
long double solve(int cnt){
sort(v+1,v+n+1,crt_b);
int i;
for(i=1;v[i].b==-1;++i);
if(n-i+1<cnt)
return -1;
int j;
long double tot=0;
int vorbitori=1;
for(j=0;j<cnt;++j){
tot+=1.0*v[i+j].b/vorbitori;
++vorbitori;
v[i+j].votat=1;
}
sort(v+1,v+n+1,crt_a);
int target=k-cnt;
for(i=1;target;++i)
if(!v[i].votat){
v[i].votat=1;
tot+=1.0*v[i].a/vorbitori;
--target;
}
for(i=1;i<=n;++i)
v[i].votat=0;
return tot;
}
long double get_sol(){
long double rsp=500000;
int i;
for(i=0;i<k;++i){
long double temp=solve(i);
if(temp!=-1 && rsp>temp)
rsp=temp;
}
return rsp;
}
int main()
{
read();
cout<<fixed<<setprecision(5)<<get_sol();
return 0;
}
