B_4948

#include <iostream>
#include <cmath>
#define number 246912
#define MAX 246913
using namespace std;

int prime_num[MAX] = {
    0,
};

void init()
{
    prime_num[1] = 0;
    for (int i = 2; i < MAX; i++)
        prime_num[i] = i;
    for (int i = 2; i <= sqrt(number); i++)
    {
        if (prime_num[i] == 0)
            continue;
        else
            for (int j = i * i; j < MAX; j += i)
                prime_num[j] = 0;
    }
}

int main(void)
{
    init();
    int n;
    while (cin >> n)
    {
        int count = 0;
        if (n == 0)
            break;
        for (int i = (n + 1); i <= (2 * n); i++)
            if (prime_num[i] != 0)
                count++;
        cout << count << '\n';
    }
}