var s:array[0..26] of longint;
    p:array[1..26] of real;
    n,k,nr:longint;
    ss:real;
procedure back(l:longint);
    var i:longint;
        pp:real;
    begin
      if l=k then
               begin
                 pp:=1;
                 for i:=1 to l do pp:=pp*p[s[i]];
                 ss:=ss+pp;
                 nr:=nr+1;
               end
            else
               begin
                 for i:=s[l]+1 to n do
                    begin
                      s[l+1]:=i;
                      back(l+1);
                    end;
               end;
    end;
begin
assign(input,'dezastru.in'); reset(input);
assign(output,'dezastru.out'); rewrite(output);
readln(n,k);
for nr:=1 to n do read(p[nr]);
ss:=0;nr:=0;
back(0);
writeln(ss/nr:9:6);
close(input);
close(output);
end.