#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n, k;
cin >> n >> k;
double ans = 1e18;
vector<pair<double, double>> s(n);
for (auto &[b, a] : s) {
cin >> a >> b;
if (b == -1)
b = 1e9;
}
ranges::sort(s);
for (int c = 0; c <= k; ++c) {
int need = k - c;
vector dp(n + 1, vector<double>(need + 1, 1e18));
dp[0][0] = 0;
for (int i = 0; i < n; ++i) {
auto [b, a] = s[i];
for (int j = 0; j <= need; ++j) {
if (dp[i][j] > 1e17)
continue;
if (j < need) {
dp[i + 1][j + 1] =
min(dp[i + 1][j + 1], dp[i][j] + double(a) / double(c + 1));
}
double cost = 0;
int use = i - j;
if (use < c)
cost = double(b) / double(use + 1);
dp[i + 1][j] = min(dp[i + 1][j], dp[i][j] + cost);
}
}
ans = min(ans, dp[n][need]);
}
cout << setprecision(12) << ans;
return 0;
}
