#include <bits/stdc++.h>
using namespace std;

const int N = 500;
const double INF = 1e18;

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    int n, k;
    cin >> n >> k;
    static array<int, 2> p[N];
    for (int i = 0; i < n; i++) {
        int a, b;
        cin >> a >> b;
        if (b == -1) b = 1e9;
        p[i] = {b, a};
    }
    sort(p, p + n);
    static int g[N][N + 1];
    for (int i = 0; i < n; i++) {
        static int t[N];
        for (int j = i; j < n; j++) t[j - i] = p[j][1];
        sort(t, t + n - i);
        g[i][0] = 0;
        for (int j = 1; j <= n - i; j++) g[i][j] = g[i][j - 1] + t[j - 1];
    }
    double ans = g[0][k];
    for (int t = 0; t <= k; t++) {
        static double f[N + 1];
        for (int i = 0; i <= t; i++) f[i] = INF;
        f[0] = 0;
        for (int i = 0; i < n; i++) {
            for (int j = t; j >= 0; j--)
                f[j] = min(f[j] + (double)p[i][1] / (t + 1),
                                     j == 0 ? INF : f[j - 1] + (double)p[i][0] / j);
            if (k - i - 1 >= 0)
                ans = min(ans, f[t] + (double)g[i + 1][k - i - 1] / (t + 1));
        }
    }
    cout << fixed << setprecision(12) << ans << '\n';
    return 0;
}