var p:array[1..26] of real;
    n,k,nr,j:longint;
    ss:real;

procedure back(l:longint;pp:real;s:longint);
var i:longint;
begin
 if l=k+1 then begin
  ss:=ss+pp;nr:=nr+1;end
          else for i:=s+1 to n-k+l do back(l+1,pp*p[i],i);end;
begin
assign(input,'dezastru.in'); reset(input);
assign(output,'dezastru.out'); rewrite(output);
readln(n,k);
 for j:=1 to n do read(p[j]);
back(1,1,0);
writeln(ss/nr:9:6);
close(input);close(output);
end.