#include <iostream>
using namespace std;

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