取整求和

$O(\sqrt{x})$时间复杂度方法：

ll s1(ll x, ll s, ll e) {
    ll i = s, res = 0;
    while (i <= min(x, e)) {
        ll v = x / i;
        ll last = x / v;
        last = min(e, last);
        res += v * (last - i + 1);
        i = last + 1;
    }
    return res;
}