const nmax=26;   
type stiva=array[1..nmax] of integer;   
var f,g:text;   
    v:array[1..nmax] of real;   
    k,x,y,l,m,p,n,i,o:longint;   
    ap:array[1..nmax] of 0..1;   
    sf,s,p1:real;   
    st:stiva;   
    q:boolean;   
procedure back(l:longint);   
var i,j:longint;   
begin  
     if l=k+1 then begin
                      p1:=1;
                      For j:=1 To k do
                      p1:=2*(p1*v[st[j]]);
                      s:=s+p1;
                      p:=p+1;
                   end
              else begin
                        For i:=st[l-1] To n do
                            begin
                                 St[l]:=i;
                                 q:=True;
                                 if ap[st[l]]=1 then q:=false;   
                                 if q then begin  
                                                ap[st[l]]:=1;   
                                                back(l+1);   
                                                ap[st[l]]:=0;   
                                           end;   
                            end;   
                   end;   
end;   
begin  
     assign(f,'dezastru.in');reset(f);   
     readln(f,n,k);   
     for i:=1 to n do  
     read(f,v[i]);   
     p:=0;   
     close(f);   
     assign(f,'dezastru.out');rewrite(f);   
     back(1);   
     sf:=s/p;   
     writeln(f,sf:0:6);   
     close(f);   
End.