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;
}