#include<stdio.h>
int c[30][30],n,k;
float v[30],a[30][30];
void dinamica()
{
	int i,j;
	c[0][1]=1;
	a[0][1]=1.0;
	k++;
	for(i=1;i<=n;++i)
		for(j=1;j<=k;++j){
			c[i][j]=c[i-i][j]+c[i-1][j-1];
			a[i][j]=a[i-1][j]+a[i-1][j-1]*v[i];
		}
	printf("%.6f",(float)a[n][k]/c[n][k]);
}
int main()
{
	int i;
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	scanf("%d %d",&n,&k);
	for(i=1;i<=n;i++)
		scanf("%f",&v[i]);
	dinamica();
	return 0;
}