题解 | 几乎是质数

#include <stdio.h>

int main(void) {
    int n;
    scanf("%d", &n);

    if (n >= 1 && n <= 5) {
        printf("0\n");
        return 0;
    }
    
    int ans = 0;
    for (int i = 6; i <= n; i++) {
        int num = 0;
        int temp = i;
        for (int j = 2; j * j <= temp; j++) {
            if (temp % j == 0){
                num++;        
                while(temp % j == 0){
                    temp = temp / j;
                }
            }
        }
        if(temp > 1)
            num++;
        if(num == 2)
            ans++;
    }
    printf("%d", ans);

    return 0;
}