#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ldb long double

const int N=2505;
ldb x[N],y[N];

//ldb fac[N][N];
ldb nCk[N][N];

ldb cross(int i,int j){
	return x[i]*y[j]-y[i]*x[j];
}

/*ldb F(int p,int q){
	if(p>=q)return fac[q][p];
	else return 1.0/fac[p][q];
	//return fac[p]/fac[q];
	//ldb ans=1;
	//for(int i=p+1;i<=q;i++)ans/=i;
	//for(int i=q+1;i<=p;i++)ans*=i;
	//return ans;
}*/
int main(){
	int n,k;
	scanf("%i %i",&n,&k);
	for(int i=0;i<n;i++){
		scanf("%Lf %Lf",&x[i],&y[i]);
	}

	/*for(int i=0;i<=n;i++){
		fac[i][i]=1;
		for(int j=i+1;j<=n;j++){
			fac[i][j]=fac[i][j-1]*j;
		}
	}*/
	for(int i=0;i<=n;i++){
		nCk[i][0]=1;
		for(int j=1;j<=i;j++){
			nCk[i][j]=nCk[i-1][j-1]+nCk[i-1][j];
		}
	}

	ldb ans=0;
	for(int i=0;i<n;i++){
		for(int sz=2;sz<=n;sz++){
			int j=(i+sz-1)%n;
			if(n-sz>=k-2){
				ans-=cross(i,j)*0.5*nCk[n-sz][k-2];//*F(n-sz,n)*F(k,k-2)*F(n-k,n-sz-k+2);
			}
		}
	}
	ans/=nCk[n][k];
	printf("%.12Lf\n",ans);
	return 0;
}