#include <bits/stdc++.h>
using namespace std;
#ifdef Ramzi
#include "debug.h"
#else
#define debug(...)
#endif
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n, k;
cin >> n >> k;
vector<pair<int, int>> a(n);
for (int i = 0; i < n; i++) {
cin >> a[i].first >> a[i].second;
}
double ans = 1e9;
for (int i = 0; i < (1 << n); i++) {
double sum = 0;
int x = 0;
multiset<pair<int, int>> s(a.begin(), a.end());
for (int j = 0; j < n; j++) {
if (i & (1 << j) && x < k) {
if (a[j].second != -1) {
x += 1;
sum += a[j].second;
s.erase(s.find(a[j]));
}
}
}
debug(sum, x);
int y = k - x;
double res = 0;
while (y--) {
res += s.begin()->first;
s.erase(s.begin());
}
sum += res / (x + 1);
ans = min(ans, sum);
}
cout << fixed << setprecision(10) << ans << "\n";
return 0;
}
