区间素数筛

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cstdlib>
typedef long long LL;
using namespace std;
const int N = 1e6+5;
bool isprime[N];
bool lowisprime[N];
void segment_even(LL x,LL y)
{
    for(int i=0;(LL)i*i<=y;i++) lowisprime[i] = true;
    for(int i=0;i<=y-x;i++) isprime[i] = true;
    for(int i=2;(LL)i*i<=y;i++)
    {
        if(lowisprime[i])
        {
            for(int j=i*2;(LL)j*j<=y;j+=i) lowisprime[j] = false;
            for(LL j=max(2LL,(x+i-1)/i)*i;j<=y;j+=i) isprime[j-x] = false;
        }
    }
}
int main()
{
    LL x,y;
    scanf("%lld %lld",&x,&y);
    segment_even(x,y);
    int cont = 0;
    for(int i=0;i<=y-x;i++) if(isprime[i]) ++cont;
    cout<<cont<<endl;
}