#include<stdio.h>

#define IN "dezastru.in","r",stdin
#define OUT "dezastru.out","w",stdout

int n,T,nr,sol[30];
double rez,rez1=1;
double prob[30];
int marc[30];

void citire()
{
	freopen(IN);
	scanf("%d%d",&n,&T);
	for(int i=1;i<=n;++i)
		scanf("%lf",&prob[i]);
}
void back(int k)
{
	if(k==T+1)
	{
		/*
		for(int i=1;i<k;++i)
			printf("%d ",sol[i]);
		printf("\n");
		*/
		rez+=rez1;
		++nr;
		return;
	}
	for(int i=sol[k-1]+1;i<=n-T+k;++i)
	{
		sol[k]=i;
		rez1*=prob[i];
		back(k+1);
		rez1/=prob[i];
	}
}
void afisare()
{
	freopen(OUT);
	printf("%.6lf\n",rez/nr);
}
int main()
{
	citire();
	back(1);
	afisare();
	return 0;
}