#include <bits/stdc++.h>
using namespace std;
#define int long long
#define vi vector<int>
#define show3(x,y,z) cout<<#x<<": "<<x<<" | "<<#y<<": "<<y<<" | "<<#z<<": "<<z<<endl;
int n,k;
vector<pair<int,int>>a;
#define ld long double
ld memo[105][1005][105];
ld dp(int pos, int collab, int left){
if(pos==n and left>0)return 1e18;
if(pos==n or left==0)return 0;
if(memo[pos][collab][left]!=-1)return memo[pos][collab][left];
ld res=2e18;
// take
// dont take collab
res=min(res,dp(pos+1,collab,left-1)+(ld)a[pos].second/(ld)collab);
//take collab
if(a[pos].first!=LLONG_MAX)
res=min(res,dp(pos+1,collab+1,left-1)+(ld)a[pos].first/(ld)collab);
// dont take
res=min(res,dp(pos+1,collab,left));
// cout<<"pos: "<<pos<<endl;
// cout<<a[pos].first<<' '<<a[pos].second<<endl;
// show3(res,collab,left);
return memo[pos][collab][left]=res;
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
cin>>n>>k;
fill(&memo[0][0][0],&memo[0][0][0]+105*1005*105,-1);
a.resize(n);
int alla=1;
for(auto &x:a){
cin>>x.second>>x.first;
if(x.first==-1)x.first=LLONG_MAX;
alla&=x.first==LLONG_MAX;
}
sort(a.begin(),a.end());
ld ans=0;
if(alla){
for(auto &x:a)swap(x.first,x.second);
sort(a.begin(),a.end());
for(int i=0;i<k;++i)ans+=a[i].first;
}
else{
ans=dp(0,1,k);
}
cout<<setprecision(15)<<ans;
}
