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 JavaScript (node.js v5.12) (judge verdict: Accepted)

function main(input) {
  if (input == 2 || input == 3) {
    console.log(input);
    return;
  }

  var base = Math.floor(
    (parseInt(input.trim()) + 1) / 6
  ) * 6;
  while (true) {
    if (isPrime(base - 1) && base - 1 >= input) {
      console.log(base - 1)
      break;
    }

    if (isPrime(base + 1) && base + 1 >= input) {
      console.log(base + 1)
      break;
    }
    base += 6;
  }
}
function isPrime(n) {
  for (var i = Math.floor(Math.sqrt(n)); i <= Math.floor(n / 2); i++) {
    if (n % i == 0) return false;
  }
  return true;
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));