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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Linq;
namespace AtCoder_Begginer_Contest1
{
    class Program
    {
        static void Main(string[] args)
        {
            int num = int.Parse(Console.ReadLine());
            int ans = 0;
            if (num == 2) ans = 2;
            else if (num % 2 == 0) num++;
            while (ans == 0)
            {
                double sqrtNum = Math.Sqrt(num);
                for (int i = 3; i <= sqrtNum; i += 2)
                {
                    if (num % i == 0)
                    {
                        // 素数ではない
                        break;
                    }
                    if (i >= sqrtNum-2) ans = num;
                }
                num+=2;
            }
            // 素数である
            Console.WriteLine(ans);
        }
    }
}