//#pragma GCC optimize("Ofast,unroll-loops,O3")
//#pragma GCC target("avx,avx2,sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,fma,tune=native")
#include<bits/stdc++.h>
//#include<bits/extc++.h>
//#pragma pack(1)
#define fast ios::sync_with_stdio(0); cin.tie(0);
#define int long long
#define pii pair<int,int>
#define x first
#define y second
#define N 515
using namespace std;
//using namespace __gnu_pbds;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
//typedef tree<int, null_type,less_equal<int>, rb_tree_tag,tree_order_statistics_node_update> order_multiset;
//typedef tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update> order_set;
int n,k,a[N],b[N];
long double dp[N],ans=1e100;
bool vis[N];
vector<pii>tog,all;
void solve(){
sort(tog.begin(),tog.end());
for (int i=0;i<=n;i++) vis[i]=0;
long double tans=0;
for (int i=0;i<tog.size();i++){
tans+=(long double)tog[i].x/(long double)(i+1);
vis[tog[i].y]=1;
}
int need=k-tog.size(),total=0;
for (int i=1;i<=n&&need;i++){
if (!vis[all[i-1].y]){
need--;
total+=all[i-1].x;
}
}
tans+=(long double)total/(long double)(tog.size()+1);
// cout<<" -> "<<tans<<'\n';
ans=min(ans,tans);
}
signed main(){
fast
cout<<fixed<<setprecision(20);
cin>>n>>k;
for (int i=0;i<n;i++){
cin>>a[i]>>b[i];
all.push_back({a[i],i});
}
sort(all.begin(),all.end());
for (int i=0;i<(1LL<<n);i++){
tog.clear();
for (int j=0;j<n;j++){
if (i&(1LL<<j)){
if (b[j]!=-1) tog.push_back({b[j],j});
}
}
if (tog.size()<=k) solve();
}
cout<<ans<<'\n';
}
