AOJ 0009 Prime Number

#include <cstdio>

int p[100010];
bool np[1000010];
int cntp;

void SievePrime(int n) {
	for (int i = 0; i <= n; ++i) np[i] = true;
	np[0] = false, np[1] = false;
	for (int i = 2; i <= n; ++i) {
		if (np[i]) {
			p[cntp++] = i;
			for (int j = 2 * i; j <= n; j += i) {
				np[j] = false;
			}
		}
	}
}

int main() {
	SievePrime(1000000);
	int n;
	while (scanf("%d", &n) != EOF) {
		int ans = 0;
		for (int i = 2; i <= n; ++i) {
			if (np[i]) ++ans;
		}
		printf("%d\n", ans);
	}
	return 0;
}