#include<stdio.h>

float p[25],a[26][26],c;
int n=0,k=0,i,j;
float x=1,y=1;



int main()
{
   freopen("dezastru.in","r",stdin);
   freopen("dezastru.out","w",stdout);
   scanf("%d %d",&n,&k);
   for(i=1;i<=n;i++)
	scanf("%f",&p[i]);
   for(i=0;i<=n;i++) a[i][0]=1;
   for(i=1;i<=n;i++)
	for(j=1;j<=n;j++)
		a[i][j]=a[i-1][j]+a[i-1][j-1]*p[i];
   for(i=k+1;i<=n;i++)
	x*=i;
   for(i=2;i<=n-k;i++)
	y*=i;
   x/=y;
   c=(float)a[n][k]/(float)x;
   printf("%.6f",c);
   return 0;
}