#include<stdio.h>
float a[32][32], pr[32];
int c[32][32];

int main()
{
	freopen("dezastru.in", "r", stdin);
	freopen("dezastru.out", "w", stdout);
	int i, j, n, k;
	float ss=1;
	scanf("%d %d", &n, &k);
	for (i=1; i<=n; i++)
		scanf("%f", &pr[i]);
	c[1][1]=1;
	for (i=2; i<=n+1; i++)
		for (j=1; j<=i; j++)
			c[i][j]=c[i-1][j]+c[i-1][j-1];
	a[0][0]=1;
	for (i=1; i<=n; i++)
		for (j=1; j<=i; j++)
			a[i][j]=a[i-1][j-1]+a[i-1][j]*pr[j];
	ss=ss/c[n+1][k+1];
	printf("%f", (a[n][k]-0.01)*ss);
	return 0;
}