1st door?

All the doors that have odd number of factors will be open at the end

They ask Puzzle. There are 100 doors in a row, all doors are initially closed. A person walks through all doors multiple times and toggle (if open then close, if close then open) them in following way: In first walk, the person toggles every door In second walk, the person toggles every second door, i.e., 2nd, 4th, 6th, 8th, … In third walk, the person toggles every third door, i.e. 3rd, 6th, 9th, … ……… ………. In 100th walk, the person toggles 100th door. Which doors are open in the end?