What is the use of -1 in  instructorAccts.push(_address) -1?
   
  

    function setInstructor(address _address, uint _age, string _fName, string _lName) public {
            var instructor = instructors[_address];
            
            instructor.age = _age;
            instructor.fName = _fName;
            instructor.lName = _lName;
            
            instructorAccts.push(_address) -1;
        }