#include<bits/stdc++.h>
using namespace std;
#define int long long
#define fi first
#define se second
#define pb push_back
#define debug(x) cout<<#x<<": "<<x<<"\n"
signed main()
{
ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
// freopen("input.000","r",stdin);
// freopen("output.000","w",stdout);
// srand((unsigned)time(NULL));
// rand()
int n,i,a[505],b[505],m,cnt,max_p,cnt0,cy[505],cn[505],size_cy,size_cn;
long double sum,ans;
bool tr;
cin>>n>>m;
for (i=1;i<n+1;i++) cin>>a[i]>>b[i];
max_p=1;
for (i=1;i<n+1;i++) max_p*=2;
max_p--;
ans=1e9;
for (cnt=0;cnt<max_p;cnt++)
{
cnt0=cnt;
size_cy=size_cn=0;tr=true;
for (i=1;i<n+1;i++)
{
if (cnt0%2==0)
{
size_cn++;
cn[size_cn]=a[i];
}
else
{
size_cy++;
cy[size_cy]=b[i];
if (b[i]==-1) tr=false;
}
cnt0/=2;
}
if (size_cy<=m and tr)
{
sort(cn+1,cn+size_cn+1);
sort(cy+1,cy+size_cy+1);
sum=0;
for (i=1;i<size_cy+1;i++) sum += (double)cy[i]/i;
for (i=1;i<=m-size_cy;i++) sum += (double)cn[i]/(size_cy+1);
ans=min(ans,sum);
}
}
cout<<fixed<<setprecision(4)<<ans;
}
