题目

阅读理解 About 30 years ago, I left Cuba for the United States with my son. After getting settled finally in Brunswick, New Jersey, I enrolled(注册) my son in kindergarten. Several weeks later, my son's teacher asked me to meet him at his office. In the teacher's office, and exchange of greetings was followed by his questions: “Is your son mentally retarded(弱智的)？Does he suffer from any kind of mental disability?” Was he talking about my wonderful Scola? NO, no, it can’t be. What a helpless, lonely moment! I told him that Scola was a quiet, sweet little boy, instead. I asked若复数（a-i）（1+i）（i是虚数单位，a∈R）是纯虚数，则a=    ．