A call from the President
In my school, there's a new transferred student. She was very welcomed by our school, because she is the President's daughter and my school makes a huge welcoming event in our school hall. At the end of the welcoming event all of the student has to shake hands with the president daughter. We are really thinks that this event is so unnecessarily, so to make fun of the teacher, we decided to shake hand with all of the students in our school.
If the number of students is 1000 (including the president daughter), and all student only shake hands with another student exactly once, how many shaking hands that happened in our hall school?