Lockers labelled from 1 to 2020 are lined up in a row. Initially, all the lockers’ doors are open. A student walks by and closes all the lockers. Another student walks by and opens all lockers whose labels are divisible by two. Yet another student walks by and switches the state of lockers (that is, the students opens closed lockers and closes opened lockers) with labels divisible by three. This process continues, with the n^th student switching the state of lockers with labels divisible by n. After 2020 students have walked by, how many lockers are closed?