#include<iomanip.h>
#include<fstream.h>
#define input "dezastru.in"
#define output "dezastru.out"
#define Nmax 30
float p[Nmax],x[Nmax],proc=1,sum=0;
int n,k;

void btk(int y,int i,float t)
     {
     if (y==k)
        sum+=t;
        
     else
         {
         for (;i<n;i++)
             {
             t*=x[i];
				 btk(y+1,i+1,t);
             t/=x[i];
             }     
         }
     }


int main()
    {
    int i;
    ifstream fin(input);
    ofstream fout(output);
    fin>>n>>k;      
    for (i=0;i<n;i++)
        fin>>x[i];
	 for(i=k;i<n;i++,proc*=i);
	 btk(0,0,1);
    sum/=proc;
    fout<<setprecision(6)<<sum<<"\n";
    return 0;
    }