#include<stdio.h>
int n,K,nr=0,i;
double P=0,a[50];
void back(int k,int poz,double S){
	if(k!=K+1){
		for(i=poz+1;i<n-K+k+1;i++)
			back(k+1,i,S*a[i]);
	}
	else{
		P+=S;
		nr++;
		return ;
	}
}
int main(){
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	scanf("%d %d",&n,&K);
	for(i=0;i<n;++i)
		scanf("%lf",&a+i);
	back(1,0,1);
	P=nr;
	printf("%.6lf",P);
	fclose(stdout);
	return 0;
}