#include <bits/stdc++.h>
using namespace std;
using ld = long double;
ld read() {
    string s;
    cin >> s;
    return stold(s);
}
ld pt[2505][2], mul[2505];
int main() {
    ios::sync_with_stdio(0); cin.tie(0);
    int n,k;
    cin >> n >> k;
    for (int i = 0; i < n; i++) {
        pt[i][0] = read();
        pt[i][1] = read();
    }
    vector<int> ind(n);
    iota(ind.begin(),ind.end(),0);
    sort(ind.begin(),ind.end(),[&](int x, int y) {
        bool ba = make_pair(pt[x][0],pt[x][1]) < pair<ld,ld>{0,0}, bb = make_pair(pt[y][0],pt[y][1]) < pair<ld,ld>{0,0};
        if (ba != bb) return ba < bb;
        return pt[x][0]*pt[y][1] > pt[x][1]*pt[y][0];
    });
    ld tot = 0;
    for (int i = 0; i <= n-k; i++) {
        ld ret = k*(k-1)/2;
        for (int j = 0; j < k-2; j++) {
            ret *= n-i-2-j;
            ret /= n-j;
        }
        ret *= 2;
        ret /= ((n-k+2)*(n-k+1));
        ld cur = 0;
        for (int j = 0; j < n; j++) {
            int l = (j+i+1)%n;
            cur += pt[ind[j]][0]*pt[ind[l]][1]-pt[ind[j]][1]*pt[ind[l]][0];
        }
        tot += ret*cur;
    }
    cout << setprecision(20) << tot/2 << '\n';
    return 0;
}