#include<stdio.h>
int n,k,i,j,cnk=1;
float a[26][26],p[26];
int main(void)
{
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	scanf("%d%d",&n,&k);
	for(i=1;i<=n;i++)
	a[i][0]=1;
	for(i=1;i<=n;i++)
		scanf("%f",&p[i]);
	a[1][1]=p[1];
	for(i=2;i<=n;i++)
		for(j=1;j<=i;j++)
			a[i][j]=a[i-1][j]+p[i]*a[i-1][j-1];
	for(i=n;i>n-k;i--)
	cnk*=i;
	for(i=1;i<=k;i++)
	cnk/=i;
	printf("%.6f",a[n][k]/cnk);
	return 0;
}