#include<stdio.h>
#include<fstream>
using namespace std;

FILE *f = fopen("dezastru.in","r");
FILE *g = fopen("dezastru.out","w");

#define MaxN 29

int N,K,nr,V[MaxN];
double S,A[MaxN],B[MaxN][MaxN];

inline void back(int k,int x,float s) //Nu merge de 100 de pct
{
	for(int i=x+1;i<=N;i++)
		if(k == K)
			S += s*A[i],++ nr;
		else
			back(k+1,i,s*A[i]);
}

void PD(void)
{
	for(int i=0;i<=N;i++)
		B[i][0] = 1;
	
	for(int i=1;i<=N;i++)
		for(int j=1;j<=K;j++)
			B[i][j] = B[i-1][j] + B[i-1][j-1]*A[i];
}

void Comb(void)
{
	for(int i=1;i<=N-K;i++)
		B[N][K] *= i;
	for(int i=K+1;i<=N;i++)
		B[N][K] /= i;
}

int main()
{
	fscanf(f,"%d%d",&N,&K);
	for(int i=1;i<=N;i++)
		fscanf(f,"%lf",&A[i]);
	
	PD();
	Comb();
	
	fprintf(g,"%.6lf\n",B[N][K]);
	
	fclose(f);
	fclose(g);
	return 0;
}