var A,B,C,K,N:longint;

function CazuriFavorabile:longint;
 var aux,i,j:longint;
 begin
     CazuriFavorabile:=0;
     i:=A;
     j:=k;
      while i<=B do
        begin
            aux:=i;
            while aux>0 do
              begin
                  if aux mod 10=C  then
                    begin
                        dec(j);
                        aux:=aux div 10;
                    end else aux:=aux div 10;
              end;
            if j<=0 then inc(cazurifavorabile);
            inc(i); j:=k;
        end;

 end;




BEGIN
      assign(input,'cifre.in'); reset(input);
      assign(output,'cifre.out'); rewrite(output);
      readln(input,A,B,C,K);
      N:=CazuriFavorabile();
      write(output,N/((B-A)+1):0:4);
      close(input);
      close(output);
END.