Problem:
Score: 300 points

Problem Statement

Find the minimum prime number greater than or equal to X.

Notes

A prime number is an integer greater than 1 that cannot be evenly divided by any positive integer except 1 and itself.

For example, 2, 3, and 5 are prime numbers, while 4 and 6 are not.

Constraints

2 \le X \le 10^5

All values in input are integers.

Input

Input is given from Standard Input in the following format:

X

Output

Print the minimum prime number greater than or equal to X.

Sample Input 1

20

Sample Output 1

23

The minimum prime number greater than or equal to 20 is 23.

Sample Input 2

2

Sample Output 2

2

X itself can be a prime number.

Sample Input 3

99992

Sample Output 3

100003

Solution in OCaml (4.02.3) (judge verdict: Accepted)

let () =
  let rec prime n m = if n = m then true else if n mod m = 0 then false else prime n (m+1) in
  let rec check n m = if prime n 2 then n else if n = m then n else check (n+1) m  in
  let n = read_int () in
  Printf.printf "%d\n" (check n (n*2))