#include <bits/stdc++.h>
#define ll long long
#define pb push_back

using namespace std;

vector<vector<double>> choose(2501, vector<double>(2501, 0));

double find_area(double x1, double y1, double x2, double y2, double x3, double y3)
{
    return abs(x1*y2+x2*y3+x3*y1-x2*y1-x3*y2-x1*y3) / 2;
}

int main()
{
    ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);
	cout << fixed << setprecision(12);

	for(int i=0; i<=2500; i++)
    {
        for(int j=0; j<=i; j++)
        {
            if(j==0) choose[i][j] = 1;
            else choose[i][j] = choose[i][j-1] * (i - j + 1) / j;
        }
    }

	int n, k;
	cin >> n >> k;

	vector<double> x(n), y(n);
	for(int i=0; i<n; i++) cin >> x[i] >> y[i];

	double ans1 = 0, S1 = 0, ans2 = 0, S2 = 0, ans3 = 0, S3 = 0;
	for(int i=1; i<n; i++)
    {
        for(int j=i+1; j<n; j++)
        {
            double A = find_area(x[0], y[0], x[i], y[i], x[j], y[j]);
            if(n-j+i-2>=k-3)
            {
                ans1 += A * choose[n-j+i-2][k-3];
                S1 += choose[n-j+i-2][k-3];
            }
            if(n-j+i-2>=k-2)
            {
                ans2 += A * choose[n-j+i-2][k-2];
                S2 += choose[n-j+i-2][k-2];
            }
            if(j-i-1>=k-2)
            {
                ans3 += A * choose[j-i-1][k-2];
                S3 += choose[j-i-1][k-2];
            }
        }
    }

    //cout << S1 << " " << S2 << " " << S3 << "\n";

    //assert(S2 / (k + 1) == S3);

    //double p1 = ans1 * (k - 2) / S1, p2 = ans2 * (k - 1) / S2 - ans3 / S3;

    //cout << p1 << " " << p2 << "\n";
    cout << (ans1  + ans2 - ans3) / (S1 / (k - 2) + S3) << "\n";


    /*for(int i=0; i<n; i++)
    {
        for(int j=i+1; j<n; j++)
        {
            for(int k=j+1; k<n; k++)
            {
                cout << i << " " << j << " " << k << " = " << find_area(x[i], y[i], x[j], y[j], x[k], y[k]) << "\n";
            }
        }
    }*/
}
/*
5 4
0 4
4 2
4 1
3 -1
-2 4
*/