#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define PII pair<int, int>
#define mkp make_pair
#define INF MAX_INT
template <typename T> inline void rd(T &x){
	x = 0; bool f = true; char ch = getchar();
	while(ch < '0' || ch > '9'){ if(ch == '-') f = false; ch = getchar();}
	while(ch >= '0' && ch <= '9'){ x = (x << 1) + (x << 3) + (ch ^ '0'); ch = getchar();}
	if(!f) x = -x;
}
template <typename T, typename ...Args> inline void rd(T &x, Args &...args){ rd(x); rd(args...);}
#define db long double
const int N = 2.5e3 + 100;
struct P {
	db x, y;
}p[N];
db times(P a, P b) {
	return (db)a.x * b.y - (db)a.y * b.x;
}
int n, k;
db C[N][N];
void init() {
	C[0][0] = 1;
	for(int i = 1; i <= n; ++i) {
		C[i][0] = 1;
		for(int j = 1; j <= i; ++j) {
			C[i][j] = C[i - 1][j] + C[i - 1][j - 1];
		}
	}
}
int main() {
    // freopen(".in", "r", stdin);
    // freopen(".out", "w", stdout);
	rd(n, k);
	init();
	for(int i = 1; i <= n; ++i) {
		scanf("%Lf%Lf", &p[i].x, &p[i].y);
	}
	db ans = 0;
	for(int i = 1; i <= n; ++i) {
		for(int len = k - 1; len < n; ++len) {
			int j = (i + len - 1) % n + 1;
			ans += times(p[i], p[j]) * C[len - 1][k - 2] / (1.00000 * C[n][k]);
		}
	}
	printf("%.8Lf", ans / 2.00000);
    return 0;
}