#include <iostream>
using namespace std;

int LietKe(int n);
int LietKe(int n)
{
	int dem = 1;
	for (int i = 1; i < n; i++)
	{
		if (n % i == 0)
			dem = dem + 1;
		
	}
	if (dem == 2)
		return 1;
	else return 0;
}
int main()
{
	int a,i,j,b,c;
	cout << "Vui long nhap a:";
	cin >> a;
	if (a == 1)
		cout << 2;
	else if (a == 2)
		cout << 3;
	else
	{
		for (i = a + 1;; i++)
			if (LietKe(i) == 1)
			{
				b = i - a;
				break;
			}
		for (j = a - 1; j > 0; j--)
			if (LietKe(j) == 1)
			{
				c = a - j;
				break;
			}
		 if (b < c)
			cout << a + b;
		else
			cout << a - c;
	}
	
	return 1;
}