#include<stdio.h>
#define MAXN 26
void precalc();
void init();
FILE*fin,*fout;
double d[MAXN][MAXN];
double val[MAXN];
int comb[MAXN][MAXN];
int N,K;
int main()
{
	fin=fopen("dezastru.in","r");
	fout=fopen("dezastru.out","w");
	fscanf(fin,"%d%d",&N,&K);
	for(int i=1;i<=N;i++)
	{
		fscanf(fin,"%lf",&val[i]);
	}
	precalc();
	fprintf(fout,"%lf",d[N][K]/comb[N][K]);
	fclose(fin);
	fclose(fout);
	return 0;
}

void precalc()
{
	//combinari
	comb[0][0]=1;
	for(int i=1;i<=N;i++)
	{
		comb[i][0]=1;
		for(int j=1;j<=i;j++)
		{
			comb[i][j]=comb[i-1][j]+comb[i-1][j-1];
		}
	}
	//dinamica
	d[1][1]=val[1];
	for(int i=2;i<=N;i++)
	{
		d[i][1]=d[i-1][1]+val[i];
	}
	for(int i=2;i<=N;i++)
	{
		for(int j=2;j<=i;j++)
		{
			d[i][j]=d[i-1][j-1]*val[i]+d[i-1][j];
		}
	}
}
void init()
{
}