#include<stdio.h>
int n,k,i,j,l,num,gata,c[26];
float s,pr,p[26];
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]);
pr=1;
for(i=1;i<=k;++i)
   {
   c[i]=i;
   pr=pr*p[c[i]];
   }
s+=pr;
num=1;
while(gata==0)
     {
     for(i=k;i>0 && (c[i]==(n-k+i));--i);
     if(i==0)
       gata=1;
     else
       {
       c[i]++;
       for(j=i+1;j<=k;++j)
          c[j]=c[j-1]+1;
       pr=1;
       for(l=1;l<=k;++l)
          pr=pr*p[c[l]];
       s+=pr;
       num++;
       }
     }
s=s/num;
printf("%.6f",s);
return 0;
}