埃拉托尼斯筛

 

 

#include<cstdio>
#include<algorithm>
#include<iostream>
#include<cstring>
using namespace std;
const int maxn = 1000010;
int pri[10000];
bool a[maxn];
int Eratosthenes_Sieve(int n, int pri[])
{
    a[0] = a[1] = 1;
    for (int i = 2; i * i <= n; i++)
    {
        if (!a[i])
        {
            for (int j = i << 1; j <= n; j += i)
            {
                a[j] = 1;
            }
        }
    }
    int cnt = 0;
    for (int i = 2; i <= n; i++)
    {
        if (!a[i])pri[cnt++] = i;
    }
    return cnt;
}
//typedef long long ll;
int main(void)
{
    long long L, R; scanf("%lld %lld", &L, &R);
    int cnt = Eratosthenes_Sieve(50000, pri);
    //cout << cnt;
    memset(a, 0, sizeof(a));
    for (int i = 0; i < cnt; i++)
    {
        for (long long j = max(2ll, ((L - 1) / pri[i] + 1)) * pri[i]; j <= R; j += pri[i])
        {
            a[j - L] = 1;
        }
    }
    if (L == 1)a[0] = 1;
    int ans = 0;
    for (long long i = L; i <= R; i++)
    {
        if (!a[i - L])ans++;
    }
    cout << ans;
    return 0;
}