#include <stdio.h>
#define nmax 30

long int c[nmax][nmax], i, n, k, j;
float sprod[nmax][nmax], v[nmax];


int main()
{
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	scanf("%ld %ld",&n, &k);
	for (i=1;i<=n;i++)
		scanf("%f",&v[i]);
	c[0][0]=1; sprod[0][0]=1;
	for (i=1;i<=n;i++)
	{
		c[i][0]=1;
		sprod[i][0]=1;
		for (j=1;j<=i;j++)
		{
			c[i][j]=c[i-1][j]+c[i-1][j-1];
			sprod[i][j]=sprod[i-1][j]+sprod[i-1][j-1]*v[i];
		}
	}
	printf("%.6lf",sprod[n][k]/c[n][k]);
	return 0;
}