#include <bits/stdc++.h>
using namespace std;
const int maxn = 21;
pair <int, int> a[maxn];
vector <int> b, c;
int main()
{
cout << setprecision(20);
int n, k;
cin >> n >> k;
if (n > 20)
{
return 0;
}
for (int i = 0;i < n;i++)
{
cin >> a[i].second >> a[i].first;
if (a[i].first == -1)
{
a[i].first = maxn * maxn * maxn;
}
}
sort(a , a + n);
long double rans = 1e18;
for (int mask = 0;mask < (1 << n);mask++)
{
if (__builtin_popcount(mask) > k)
{
continue;
}
for (int i = 0;i < n;i++)
{
if (mask & (1 << i))
{
b.push_back(a[i].first);
}
else
{
c.push_back(a[i].second);
}
sort(c.begin(), c.end());
}
long double ans = 0;
int tmp = 1;
for (auto o : b)
{
ans += (long double)o / tmp;
tmp++;
}
for (int i = 0;i < k - __builtin_popcount(mask);i++)
{
ans += (long double)c[i] / tmp;
}
rans = min(rans, ans);
b.clear();
c.clear();
}
cout << rans;
}
