区间埃筛

我们可以发现L,R过大，可R - L 只有1e6
所以区间筛
废话不多，上代码

#include <iostream>
#include <cmath>
using namespace std;
bool is_s[1000005], is_seg[1000005];
int sp[1000005], sc = 0;
int main() {
    long long L, R;
    cin >> L >> R;
    int lim = sqrt(R) + 1;
    for (int i = 2; i <= lim; ++i) {
        if (!is_s[i]) {
            sp[sc++] = i;
            if ((long long)i * i <= lim)
                for (int j = i * i; j <= lim; j += i) is_s[j] = 1;
        }
    }
    int len = R - L + 1;
    for (int i = 0; i < len; ++i) is_seg[i] = 1;
    for (int k = 0; k < sc; ++k) {
        long long p = sp[k];
        long long start = L % p ? L + p - L % p : L;
        if (start < p * 2) start = p * 2;
        for (long long j = start; j <= R; j += p) is_seg[j - L] = 0;
    }
    int cnt = 0;
    for (int i = 0; i < len; ++i)
        if (is_seg[i] && L + i != 1) cnt++;
    cout << cnt << endl;
    return 0;
}