#include <stdio.h>
double s=0, f[26];
int n, x, k, o=0;

void back(int k, int l, double z)
	{
	if (k==x)
		{
		o++;
   	s+=z;
		return;
		}
   for (int i=l+1;i<=n-x+k;i++)
		back(k+1, i, z*f[i]);
	}

int main(void)
{
freopen ("dezastru.in", "r", stdin);
freopen ("dezastru.out", "w", stdout);
scanf("%d%d", &n, &x);
for (register int i=0;i<n;i++)
	scanf ("%lf", &f[i]);
back(0, -1, 1);
printf ("%.6lf", s/o);
return 0;
}