Задача 13.

(the Sieve of Eratosthenes) Numbers from $2$ to $1000$ are written on a blackboard. Eratosthenes circles number $2$ and erases all numbers other than $2$ which are divisible by two. Then he repeats the process: he circles the smallest not yet circled number and erases all others which are divisible by it (doesn't matter if they are circled already). He stops once only circled numbers are left on the blackboard. What numbers are left on the blackboard?