#include<bits/stdc++.h>

#define double long double

using namespace std;

const int N = 4005;

int n,m;

double x[N],y[N];

int p[N];

bool cmp(int xx,int xy)

{

	return y[xx]<y[xy];

}

double b[N];

int main()

{

	scanf("%d%d",&n,&m);

	for(int i=1;i<=n;i++)

	{

		scanf("%Lf%Lf",&x[i],&y[i]);

		x[i]/=1000;y[i]/=1000;

	}

	for(int i=1;i<=n;i++)p[i]=i;

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

	b[0]=1;

	double ans=0;

	double now=1;

	for(int i=1;i<=n;i++)

	{

		int t=p[i];

		if(m-(n-i+1)*2>=i-1)

		{

			double tmp=0;

			for(int j=i;j<=n;j++)

			{

				tmp+=1/y[p[j]];

			}

			ans+=b[m-(n-i+1)*2]*tmp;

			now-=b[m-(n-i+1)*2];

			b[m-(n-i+1)*2]=0;

		}

		ans+=now/(x[t]+y[t]);

		for(int j=2*n;j>=0;j--)

		{

			b[j]=0;

			if(j>=1)b[j]=b[j-1]*x[t]/(x[t]+y[t]);

			if(j>=2)b[j]+=b[j-2]*y[t]/(x[t]+y[t]);

		}

	}

	printf("%.10Lf\n",ans);

	return 0;

}