#include <bits/stdc++.h>
#define INF 1000000000
#define N 504
using namespace std;
typedef long long ll;

int n, K;
struct Pnt {
    int a, b;
    bool operator< (const Pnt &x) const { return b<x.b || (b==x.b && a>x.a); }
} p[N];

int suf[N][N];
double f[N];

int main ()
{
    scanf("%d %d", &n, &K);
    for (int i=1; i<=n; i++) {
        scanf("%d %d", &p[i].a, &p[i].b);
        if (p[i].b==-1) { p[i].b=INF; }
    }

    sort(p+1, p+n+1);

    suf[n+1][1]=INF;
    for (int i=n; i>=1; i--) {
        for (int j=1; j<=n-i+1; j++) {
            suf[i][j]=min(suf[i+1][j], suf[i+1][j-1]+p[i].a);
        }
        suf[i][n-i+2]=INF;
    }

    // cout<<fixed<<setprecision(15);

    double ans=INF;

    for (int cur=0; cur<=K; cur++) {

        f[0]=0; for (int j=1; j<=cur; j++) { f[j]=INF; }

        double sum=0;
        double res=sum+f[cur]+((double)suf[1][K-cur]/(double)(cur+1));
        ans=min(ans, res);

        for (int i=1; i<=min(K, n-(K-cur)); i++) {
            sum+=p[i].a;

            for (int j=cur; j>=0; j--) {
                if (j && p[i].b<INF) { f[j]=min(f[j], f[j-1]+((double)p[i].b/(double)j)-((double)p[i].a/(double)(cur+1))); }
            }

            res=f[cur]+((double)(sum+suf[i+1][K-i])/(double)(cur+1));
            ans=min(ans, res);
        }
    }

    printf("%.15lf\n", ans);

    return 0;
}