#include <bits/stdc++.h>
using namespace std;
const int N=505,M=1005;
int n,K;

namespace solve1{
	struct nd{int a,b;} s[N];
	int a[N],b[N],A,B;
	double ans = 2e9;
	void main(){
		for(int i=1;i<=n;i++){
			scanf("%d%d",&s[i].a,&s[i].b);
		}
		for(int S=0;S<(1<<n);S++){
			A=B=0;
			bool flg=1;
			for(int i=1;i<=n;i++){
				if(S & (1<<(i-1))){
					if(s[i].b==-1) flg=0;
					b[++B] = s[i].b;
				}
				else{
					a[++A] = s[i].a;
				}
			}
			if(!flg || B>K) continue;
			double res=0;
			sort(a+1,a+A+1);
			sort(b+1,b+B+1);
			for(int i=1;i<=B;i++)
				res += (double)b[i]/i;
			for(int i=1;i<=min(K-B,A);i++)
				res += (double)a[i]/(B+1);
			ans = min(res,ans);
		}
		printf("%.10lf\n",ans);
	}
}
namespace solve2{
	int A,B;
	struct nd{int val,id;
		bool operator < (const nd &y) const{
			return val<y.val;
		}
	} a[N],b[N];
	bool vis[N];
	double ans=1e9;
	void main(){
		for(int i=1;i<=n;i++){
			int x,y;
			scanf("%d%d",&x,&y);
			a[++A] = nd{x,i};
			if(y!=-1) b[++B] = nd{y,i};
		}
		sort(a+1,a+A+1);
		sort(b+1,b+B+1);
		double base=0,res=0;
		for(int i=0;i<=B;i++){
			if(i) base += (double)b[i].val/i;
			vis[b[i].id] = 1;
			res = base;
			for(int j=1,cnt=i;cnt<K;j++){
				if(vis[a[j].id]) continue;
				res += (double)a[j].val/(i+1);
				cnt++;
			}
			ans = min(res,ans);
		}
		printf("%.10lf\n",ans);
	}
}
int main(){
	scanf("%d%d",&n,&K);
	if(n<=20) solve1::main();
	else solve2::main();
	return 0;
}