#include "bits/stdc++.h"
#define rep(i, a, n) for (auto i = a; i <= (n); ++i)
#define revrep(i, a, n) for (auto i = n; i >= (a); --i)
#define all(a) a.begin(), a.end()
#define sz(a) (int)(a).size()

using namespace std;

template<class A> string to_string(A v) {
	bool first = 1;
	string res = "{";
	for (const auto &x: v) {
		if (!first) res += ", ";
		first = 0;
		res += to_string(x);
	}
	res += "}";
	return res;
}

void debug_out() { cerr << endl; }
template<class Head, class... Tail> void debug_out(Head H, Tail... T) {
	cerr << " " << to_string(H);
	debug_out(T...);
}
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)

using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;
using vvi = vector<vi>;

template<class T>
struct Point {
	using P = Point;

	T x, y;
	P operator -(P b) const { return P{x - b.x, y - b.y}; }
	T cross(P b) const { return x * b.y - y * b.x; }
};


int main() {
	ios::sync_with_stdio(0); cin.tie(0);

	using T = long double;
	using P = Point<T>;

	const int maxn = 2500;
	vector<vector<T>> binom(maxn + 1, vector<T>(maxn + 1));
	rep(i, 0, maxn) {
		binom[i][0] = 1;
		rep(j, 1, i) binom[i][j] = binom[i - 1][j - 1] + binom[i - 1][j];
	}
	debug(binom[maxn - 1][maxn / 2] / binom[maxn - 1][maxn / 2]);

	int n, k; cin >> n >> k;
	vector<P> as(n);
	for (auto &[x, y]: as) cin >> x >> y;


	T ans = 0;
	rep(i, 0, n - 1) {
		P a = as[i];
		P b = as[(i + 1) % n];
		ans += b.cross(a) / 2;
	}
	//debug(ans);
	
	rep(i, 0, n - 1) {
		P a = as[i];
		T sum = 0;
		rep(j, 1, n - 1) {
			P b = as[(i + j - 1) % n];
			P c = as[(i + j) % n];
			int cnt = n - (j + 1);
			if (n < k - 2) break;
			sum += (c - a).cross(b - a) / 2;
			ans -= binom[cnt][k - 2] / binom[n][k] * sum;
		}
	}
	
	printf("%.10f\n", (double) ans);
	return 0;
}