#include <stdio.h>
FILE *f;long double a[26],q,s,ss;long long i,v[26],z1,l1,k,n;
int main()
{f=fopen("dezastru.in","r");fscanf(f,"%ld%ld",&n,&k);
 for(i=1;i<=n;i++)fscanf(f,"%Lf",&a[i]);fclose(f);
 l1=1;v[1]=0;
 while(l1)
 {z1=1;if(v[l1]<n)v[l1]++;else z1=0;
  if(z1)if(k==l1){s=1;for(i=1;i<=k;i++)s*=a[v[i]];ss+=s;}
	    else {l1++;v[l1]=v[l1-1];}
	else l1--;
 }
 q=ss;for(i=1;i<=n-k;i++)q=q/((k+i)/i);
 f=fopen("dezastru.out","w");fprintf(f,"%.6Lf",q);fclose(f);
 return 0;
}