C++ :

#include <algorithm>
#include <cstdio>

using namespace std;

const int N = 1e5 + 5;

int n, p[N], cnt;
bool np[N];

int main() {
    scanf("%d", &n);
    for (int i = 2; i <= n; i++) {
        if (!np[i]) p[++cnt] = i;
        for (int j = 1; j <= cnt && i * p[j] <= n; j++) {
            np[i * p[j]] = 1;
            if (i % p[j] == 0) break;
        }
    }
    printf("%d\n", 1 + cnt);
    return 0;
}