#include<stdio.h>
FILE *f=fopen("dezastru.in","r");
FILE *g=fopen("dezastru.out","w");
int n,k,nr,v[100],a[100];
float s,b[100];
void suma(void)
{
float p=1;
int i;
for(i=1;i<=k;i++)
p*=b[a[i]];
s+=p;
nr++;
}
void bec(int i)
{
int j;
if(i>k)
suma();
else
for(j=a[i-1]+1;j<=n;j++)
if(!v[j])
{
v[j]=1;
a[i]=j;
bec(i+1);
v[j]=0;
}
}
int main(void)
{
int i;
fscanf(f,"%ld%ld",&n,&k);
for(i=1;i<=n;i++)
fscanf(f,"%f",&b[i]);
bec(1);
fprintf(g,"%f",s/nr);
fclose(f);
fclose(g);
return 0;
}