#include <fstream.h>
#include <stdio.h>


int sol[30],n,kk;
long nr;
double  fact,fact2;
double v[30],st,p,sm;

/*int cond(int a){
	for (int i=0;i<a;i++)
		if (sol[i]==sol[a]) return 0;
	return 1;
}

double calc(int a){
	 double p=1;
	 for (int i=1;i<=a;i++)
		 p*=v[sol[i]];
	 return p;

	 }
*/

int main(){
	ifstream f("dezastru.in");
	freopen("dezastru.out","w",stdout);
	int k;
	f>>n;
	int i;
	f>>kk;
	for ( i=1;i<=n;i++) f>>v[i];
	v[n+1]=1;v[0]=1;
	k=1;sol[k]=1;p=v[sol[k]];
	while (k>0){
		  if (sol[k]>n-(kk-k)) {k--;p/=v[sol[k]]; sol[k]++;
                                if (sol[k]<=n-(kk-k)) p*=v[sol[k]];
                                } 
		  else if (k==kk) {/*st+=calc(k);*/sm+=p;sol[k]++;nr++;
                           p/=v[sol[k]-1];
                           if (sol[k]<=n-(kk-k)) p*=v[sol[k]];
                           }
		  else if (k<n) {sol[++k]=sol[k-1]+1; p*=v[sol[k]];}
		  }

    //st/=nr;
	sm/=nr;
	printf("%lf",sm);

	f.close();
	return 0;
}