#include <stdio.h>
float s=0, f[26], p;
int n, x, k, st[20], o=0;
void back(int k, int d, double p)
	{
   if (k==x)
   	{
      o++;
      s+=p;
      return;
      }
   for (int i=d+1;i<=n-x+k;i++)
   	back(k+1, i, p*st[i]);
   }
int main(void)
{
freopen ("dezastru.in", "r", stdin);
freopen ("dezastru.out", "w", stdout);
scanf("%d%d", &n, &x);
for (register int i=1;i<=n;i++)
	scanf ("%f", &f[i]);
back(0, -1, 1);
printf ("%.6f", s/o);
return 0;
}