program p_058;  {cifre}
var a,b,i,t,k,s:longint;
    c,nr:integer;
    p:real;
begin
  assign(input,'cifre.in'); reset(input);
  assign(output,'cifre.out'); rewrite(output);
  readln(a,b,c,k);
  for i:=a to b do begin
    nr:=0;t:=i;
    while t<>0 do begin
      if t mod 10=c then nr:=nr+1;
      t:=t div 10;
    end;
    if nr>=k then p:=p+1;
  end;
  s:=(b-a)+1;
  p:=p/s;
  write(p:4:4);
  close(input); close(output);
end.