#include<iostream>
#include<stdio.h>
#define f(i,a,b) for(i=a;i<=b;i++)
FILE *f=fopen("dezastru.in","r"),*g=fopen("dezastru.out","w");
float a[30];double aux=1,rez;
double b[30][30];
int main()
{
	int i,j,i1,n,k,aux;
	double nr;b[0][0]=1;
    fscanf(f,"%d%d",&n,&k);
	f(i,1,n)
		fscanf(f,"%f",&a[i]),b[0][i]=1;
	f(i,1,k)
		f(j,1,n)
		  b[i][j]=b[i-1][j-1]*a[j]+b[i][j-1];
	aux=n-k;nr=b[k][n];
	f(i,2,aux)
		nr*=i;
	f(i,k+1,n)
		nr/=i;
    fprintf(g,"%6lf",nr);
    return 0;
}