#include <bits/stdc++.h>
using namespace std;
#define int int64_t
int n, k;
vector<int> a, b;
signed main() {
cin.tie(0);
ios_base::sync_with_stdio(0);
cin >> n >> k;
a.assign(n, 0);
b.assign(n, 0);
vector<pair<int, int>> blub(n);
for (int i = 0; i<n; i++) {
cin >> blub[i].second;
cin >> blub[i].first;
if (blub[i].first == -1) blub[i].first = 1e18;
}
sort(blub.begin(), blub.end());
for (int i=0; i<n; i++) {
a[i] = blub[i].second;
b[i] = blub[i].first;
}
vector<array<array<double, 505>, 505>> dp(505, array<array<double, 505>, 505>());
for (int i = 0; i<505; i++) {
for (int j = 0; j<505; j++) {
for (int k = 0; k<505; k++) {
dp[i][j][k] = 1e18;
}
}
}
dp[0][0][0] = 0;
for (int i = 0; i<n; i++) {
for (int votes = 0; votes<=n; votes++) {
for (int colab = 0; colab<=n; colab++) {
array<double, 3> options={1e18, 1e18, 1e18};
options[0] = (dp[i][votes][colab]);
if (votes > 0) options[1] = (dp[i][votes-1][colab] + ((double)a[i]/(colab+1)));
if (votes > 0 && colab > 0 && b[i] != 1e18) options[2] = (dp[i][votes-1][colab-1] + ((double)b[i]/colab));
dp[i+1][votes][colab] = *min_element(options.begin(), options.end());
// cerr << dp[i+1][votes][colab] << "\n";
}
}
}
cout.precision(100);
cout << *min_element(dp[n][k].begin(), dp[n][k].end()) << "\n";
return 0;
}
