[백준 4948] 베르트랑 공준 (Eratos) (C/C++)

#include <stdio.h>
#include <iostream>
#include <algorithm>
using namespace std;
 
int N;
int num[250000];
 
void eratos(int N)
{
    for(int i = 2; i <= N; i++)
    {
        if(num[i] == 1)
        {
            continue;
        }
        
        for(int j = i+i; j <= N; j += i)
        {
            num[j] = 1;
        }
    }
}
 
int main(void)
{
//    freopen("B4948_input.txt", "r", stdin);
    
    num[1] = 1;
    eratos(250000);
    
    while(cin >> N)
    {
        int cnt = 0;
        
        if(N == 0)
        {
            break;
        }
        
        for(int i = N+1; i <= N+N; i++)
        {
            if(num[i] == 0)
            {
                cnt++;
            }
        }
        
        cout << cnt << "\n";
    }
    
    return 0;
}