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

procedure back(l:longint;pp:real;s:longint);
    var i:longint;
    begin
      if l=k then begin
                    ss:=ss+pp;
                    nr:=nr+1;
                  end
            else for i:=s+1 to n do back(l+1,pp*p[i],i);
    end;

begin
assign(f{input},'dezastru.in'); reset(f{input});
assign(g{output},'dezastru.out'); rewrite(g{output});
readln(f,n,k);
for nr:=1 to n do read(f,p[nr]);
ss:=0;nr:=0;
back(0,1,0);
writeln(g,ss/nr:9:6);
close(f{input});
close(g{output});
end.