#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 Citire()
{int i;
f>>n>>m;
for (i=1;i<=n;i++) f>>a[i];
}
inline void Back(int k,double sum)
{int i;
if (k>m) {sol+=sum;nrsol++;}
else for (i=poz[k-1]+1;i<=n-m+k;i++) {poz[k]=i;Back(k+1,sum*a[i]);poz[k]=0;}
}
int main()
{
Citire();
Back(1,1.0);
sol/=nrsol;
g<<sol;
return 0;
}