#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],aux=1,rez,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,"%f",nr);
    return 0;
}