C++ :

#include<iostream>
#include<cstdio>
#include<algorithm>
#include <cmath>
#define N 10005
using namespace std;

int w[N],v[N];
double tmp[N];
int n,k;
double tryit(double m)
{
	for(int i=0;i<n;i++)
        tmp[i] = v[i] - w[i]*m;
	sort(tmp,tmp+n);
	double sum = 0;
	for(int i=n-1;i>=n-k;i--)
        sum += tmp[i];
	return sum;
}
int main()
{
	while(~scanf("%d %d",&n,&k)){
		double t,l = 0,r = 0;
		for(int i=0; i<n; i++){
			scanf("%d %d",&w[i],&v[i]);
			t = v[i]*1.0/w[i];
			r = t > r ? t : r;
		}
		while(fabs(r-l)>1e-4){
			double m = (r+l)/2;
			if(tryit(m)>=0)
                l = m;
			else r = m;
		}
		printf("%.2lf\n",l);
	}
	return 0;
}