#include <fstream>
using namespace std;
ifstream f("dezastru.in"); ofstream g("dezastru.out");
double a[26],sum=1,sol;
int n,m,poz[26],nrsol=0;
void Back(int k,double sum)
{if (k>m) {sol+=sum;nrsol++;}
else for (int i=poz[k-1]+1;i<=n-m+k;i++) {poz[k]=i;Back(k+1,sum*a[i]);poz[k]=0;}
}
int main()
{
f>>n>>m;
for (int i=1;i<=n;i++) f>>a[i];
Back(1,1.0);
g<<(1.0*sol)/(1.0*nrsol);
return 0;
}