#include<cstdio>
using namespace std;
FILE *f,*g;
int n,v[30],p,cont;
float a[30],x=1,s=0;
//inline void edit(){for(int i=1;i<=n;++i) fprintf(g,"%d ",v[i]); fprintf(g,"\n");}
inline void edit(){for(int i=1;i<=p;++i) x*=a[v[i]]; s+=x,x=1;cont++;}
inline bool cond(int ok){for(int i=1;i<ok;++i)	if(v[i]==v[ok]) return 0; return 1;}
void back()
{
int k=1; v[k]=0;
do{ while(v[k]<n) { v[k]++; if(cond(k))  {if(k==n) edit(); else v[++k]=0;}}
   k--;
  }while(k>0);
}
int main()
{
f=fopen("dezastru.in","r");
g=fopen("dezastru.out","w");
fscanf(f,"%d %d",&n,&p);
for(int i=1;i<=n;++i) fscanf(f,"%f",&a[i]);
back();
fprintf(g,"%f\n",s/cont);
return 0;
}