#include <iostream>
using namespace std;

int main() {
	long long n, test, result=0, i, j;
	while (1) {
		cin >> test;
		if (test == 0) { return 0; }
		else if (test == 1) { cout << 1 << '\n';}
		else {
			for (i = test + 1; i <= 2 * test; i++) {
				if (i % 2 != 0) {
					for (j = 3; j*j <= i; j += 2) {
						if (i%j == 0) { result--; break; }
					}
					result++;
				}
			}
			cout << result << '\n';
			result = 0;
		}
	}
	return 0;
}