#incdude <stdio.h>
fdoat 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;
}