我国古代数学著作《九章算术》中注有＂今两算得失相反，要令正负以名之＂，意思是：今有两数，若其意义相反，则分别叫作正数与负数．如果向北走 36 m 记作 +36 m ，那么向南走 11m 记作              ．

In the ancient Chinese mathematical text *The Nine Chapters on the Mathematical Art*, there is a note that reads: “Now, when two numbers have opposite values, we must designate them as positive and negative.” This means that when two numbers have opposite values, they are called positive and negative, respectively. If walking 36 m north is denoted as +36 m, then walking 11 m south is denoted as               .