#include<stdio.h>
int n,k;
int c[32][32];
double v[32];
double d[32][32];

void read()
{
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	scanf("%d%d",&n,&k);
	int i;
	for(i=1;i<=n;i++)
		scanf("%lf",&v[i]);
}

void rez()
{
	int i,j;
	for(i=0;i<=n;i++)
		d[i][0]=1;
	for(i=1;i<=n;i++)
		for(j=1;j<=k;j++)
			d[i][j]=d[i-1][j]+d[i-1][j-1]*v[i];

	for(i=1;i<=n;i++)
	{
		c[i][1]=i;
		for(j=2;j<=k;j++)
			c[i][j]=c[i-1][j-1]+c[i-1][j];
	}
	printf("%lf\n",d[n][k]/c[n][k]);
}

int main()
{
	read();
	rez();
	return 0;
}