#include<fstream.h>
#include<iomanip.h>
#define NMAX 32

int nr[NMAX][NMAX], n, k, i, j, l, pos;
double a[NMAX][NMAX], rez=0;

ifstream f("dezastru.in");
ofstream g("dezastru.out");

int main()
{
	f>>n>>k;
	for (i=1; i<=n; ++i) nr[1][i]=1, f>>a[1][i];
	for (i=2; i<=k; ++i)
		for (j=i; j<=n; ++j)
			for (l=i-1; l<j; ++l)
			{
				a[i][j]+=a[i-1][l]*a[1][j];
				nr[i][j]+=nr[i-1][l];
			}
	for (i=k; i<=n; ++i) 
	{
		rez+=a[k][i];
		pos+=nr[k][i];
	}
	g<<fixed<<setprecision(7)<<rez/(double)pos<<"\n";
	f.close();
	g.close();
	return 0;
}