program jiji;
var f:text;
    a,b,c,k,x,i,m,nr:longint;
    p:real;
begin
  assign(f,'cifre.in');
  reset(f);
  readln(f,a,b,c,k);
  close(f);
  assign(f,'cifre.out');
  rewrite(f);
  nr:=0;
  for i:=a to b do
    begin
      m:=0;
      x:=i;
      while x<>0 do begin
                      if x mod 10=c then m:=m+1;
                      x:=x div 10
                    end;
     if m>=k then nr:=nr+1;
   end;
   p:=nr/b-a+1;
   write(f,p:6:4);
   close(f);
end.