#include<fstream>
#include<cstdio>
using namespace std;
FILE *f,*g;
int n,v[30],p;
float a[30],x=1,s=0;
inline void edit(){x=1; for(int i=1;i<=p;++i)x*=a[v[i]];s=s+x;}
inline bool cond(int ok){for(int i=1;i<ok;++i) if(v[i+1]<=v[i]) return 0; return 1;}
void back()
{
int k=1; v[k]=0;
do{ while(v[k]<n) { v[k]++; if(cond(k))  {if(k==p) edit(); else {++k;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();
int fg=1;
for(int i=2;i<=n;++i) fg*=i;
fprintf(g,"%f\n",s/fg*2);
return 0;
}