#include <bits/stdc++.h>

#define fopi freopen("in.txt", "r", stdin)

#define fopo freopen("out.txt", "w", stdout)

using namespace std;

typedef long long LL;

typedef long double ld;

const int maxn =  + ;

struct Point {

    ld x, y;

}a[maxn];

ld C[maxn][maxn];

void getC(int n) {

    C[][] = ;

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

        C[i][] = ;

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

            C[i][j] = C[i-][j] + C[i-][j-];

    }

}

ld Cross(Point a, Point b) {

    return a.x*b.y - a.y*b.x;

}

int n, k;

int main() {

    ios::sync_with_stdio(false);

    cin >> n >> k;

    getC(n);

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

        cin >> a[i].x >> a[i].y;

    ld ans = ;

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

    for (int j = k-; j <= n-; j++) {

        int t = i+j;

        if (t > n) t -= n;

        ans += Cross(a[i], a[t]) * C[j-][k-] / C[n][k];

    }

    printf("%.7Lf\n", ans/);

}