var a,b,i,c,k,s,t,nr:longint;
begin
assign(input,'cifre.in');reset(input);
assign(output,'cifre.out');rewrite(output);
read(a,b,c,k);
 for i:=a to b do begin
  t:=i;nr:=0;
  while t<>0 do begin
   if t mod 10=c then inc(nr);
   t:=t div 10;
  end;
  if nr>=k then inc(s);
 end;
writeln(s/(b-a+1):4:4);
close(input);close(output);
end.