length of string result not as expected

[jicman]

Greetings.
import std.stdio;

void main()
{
  char[] str = "不良反應事件和產品客訴報告"; // 13 chinese characters...
  writefln(str.length);
}

this program returns 39.  I expected to return 13.  How do I know 
the exact length of the characters that I have in a char[] 
variable?  Thanks.

josé