[4948] 베르트랑 공준

#include <iostream>
using namespace std;
int main()
{
	bool arr[246913] = { 0 };
	int i, j;
	arr[0] = arr[1] = true;
	for (i = 2; i <= 246913; i++)
	{
		for (j = 2; i * j <= 246913; j = j + i)
		{
			if(!arr[i*j])
				arr[i*j] = true;
		}
	}
	while (1)
	{
		int num, count = 0;
		
		cin >> num;
		
		if (num == 0)
			break;
		for (i = num+1; i <= 2 * num; i++)
		{
			if (!arr[i])
				count++;
		}
		cout << count << endl;
	}
	return 0;
}