AIZU 0009

Prime Number

Time Limit : 1 sec, Memory Limit : 65536 KB
Japanese version is here

Prime Number

Write a program which reads an integer n and prints the number of prime numbers which are less than or equal to n. A prime number is a natural number which has exactly two distinct natural number divisors: 1 and itself. For example, the first four prime numbers are: 2, 3, 5, 7.

Input

Input consists of several datasets. Each dataset has an integer n (n ≤ 999999) in a line.

The number of datasets ≤ 30.

Output

For each dataset, prints the number of prime numbers.

Sample Input

10
3
11

Output for the Sample Input

4
2
5

水题，素数筛选。

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <iostream>

using namespace std;

#define maxn 999999 + 5
typedef long long ll;

int prime[maxn];

void init() {
        for(int i = 2; i <= maxn - 5; i++) {
                 prime[i] = i % 2;
        }

        prime[2] = 1;

        for(int i = 2; i * i <= maxn; i++) {
                if(!prime[i]) continue;
                for(int j = i; j * i <= maxn; j++) {
                        prime[j * i] = 0;
                }
        }

        for(int i = 1; i <= maxn - 5; i++) {
                prime[i] += prime[i - 1];
        }
}

int main() {
        int n;
        init();
        while( ~scanf("%d",&n)) {
                printf("%d
",prime[n]);
        }

        return 0;


}