#include<stdio.h>
int st[25],i,prod,n,m;
float suma,a[25];

void back(int k,float p)
{int i;
     if (k==m+1) suma+=p;
     else
     {
	 for (i=st[k-1]+1;i<=n;i++)
	     st[k]=i,back(k+1,p*a[i]);
     }
}

int main()
{
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf("%d %d",&n,&m);
    for (i=1;i<=n;i++)
	scanf("%f",&a[i]);
    prod=1;
    for (i=m+1;i<=n;i++)
	prod*=i;
    back(1,1);
    printf("%.6f",(float)suma/prod);
    return 0;
}