MAGIC SQUARE: Calculate A+B-C

According to a news report, a certain private school in Washington recently was faced with a unique problem. A number of 12-year-old girls were beginning to use lipstick and would put it on in the bathroom. That was fine, but after they put on their lipstick they would press their lips to the mirror leaving dozens of little lip prints. Every night,the maintenance man would remove them and the next day, the girls would put them back. Finally the principal decided that something had to be done. She called all the girls to the bathroom and met them there with the maintenance man.... She explained that all these lip prints were causinga major problem for the custodian who had to clean the mirrors every night. To demonstrate how difficult it had been to clean the mirrors, she asked the maintenance man to show the girls how much effort was required. He took out a long-handled squeegee, dipped it in the toilet, and cleaned the mirror with it. Since then, there have been no lip prints on the mirror. There are teachers, and then there are educators...