#include<bits/stdc++.h>
using namespace std;
#define N 5050
struct P
{
	double x,y;
	void read(){scanf("%lf%lf",&x,&y);}
	double operator *(const P &t)const{return x*t.y-y*t.x;}
}p[N];
int n,k;
double c[N],ans;
int main()
{
	scanf("%d%d",&n,&k);
	for(int i=1;i<=n;i++)p[i].read(),p[i+n]=p[i];
	for(int i=1;i<=n;i++)ans+=p[i]*p[i+1];
	ans=fabs(ans);
	c[1]=1.0*k*(k-1)/n/(n-1);
	for(int i=2;i<=n-k+2;i++)c[i]=c[i-1]*(n-i-k+2)/(n-i);
	for(int i=1;i<=n;i++){
		double s=p[i]*p[i+1],nw;
		for(int l=2;l<=n-k+2;l++){
			int j=i+l;
			s+=p[j-1]*p[j];
			nw=fabs(s+p[j]*p[i]);
			ans-=c[l]*nw;
		}
	}
	printf("%.6f\n",0.5*ans);
}