#include<bits/stdc++.h>
using namespace std;
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int n, k;
cin >> n >> k;
vector<pair<double, double>> v(n);
for (int i = 0; i < n; i++) {
cin >> v[i].first >> v[i].second;
if (v[i].second == -1) v[i].second = 1e9;
}
sort(v.begin(), v.end());
double ans = 2e18;
for (int c = 0; c < k; c++) {
int mx = k - c;
vector<double> dp(mx + 1, 1e18);
dp[0] = 0;
for (int i = 0; i < n; i++) {
for (int e = mx; e >= 0; e--) {
if (e != mx) dp[e + 1] = min(dp[e + 1], dp[e] + v[i].first / (1.0 + c));
int id = i + 1 - e;
if (id <= 0) continue;
if (id <= c) dp[e] += v[i].second / (double)id;
}
}
ans = min(ans, dp[mx]);
}
cout << fixed << setprecision(9) << ans << "\n";
return 0;
}
