筛法求素数

#include <iostream>
#include<math.h>
using namespace std;
typedef  long long ll;
#define maxn 10000000
bool IsPrime[maxn];
void init()
{
    ll i = 2;
    for(i;i < maxn; i ++)
    {
        IsPrime[i] = true;
    }
    IsPrime[0] = IsPrime[1] = false;
}
int main()
{
    ll a,b,sum;
    init();
    while(cin >> a >> b)
    {
        ll i,j;
        for(i = 1;i <= a;i ++)
        {
            if(IsPrime[i] == true)
            {
                for(j = 2;i*j <= b;j ++)
                    IsPrime[i*j] = false;
            }
        }
        ll ans = 0;
        for(i = a;i <= b;i ++)
        {
            if(IsPrime[i])
                ans ++;
        }
        cout << ans << endl;
    }
    return 0;
}