E - Fraction Floor Sum

• Time : O(sqrt(n))
• Space : O(1)

#include <bits/stdc++.h>

#pragma optimization_level 3
#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math,O3")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx")
#pragma GCC optimize("Ofast")//Comment optimisations for interactive problems (use endl)
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimization ("unroll-loops")

using namespace std;

struct PairHash {
    inline std::size_t operator()(const std::pair<int, int> &v) const { return v.first * 31 + v.second; }
};

// speed
#define Code ios_base::sync_with_stdio(false);
#define By ios::sync_with_stdio(0);
#define Sumfi cout.tie(NULL);

// alias
using ll = long long;
using ld = long double;

// constants
const ld PI = 3.14159265358979323846;  /* pi */
const ll INF = 987654321;
const ld EPS = 1e-9;
const ll MAX_N = 202020;

// typedef
typedef pair<ll, ll> pll;
typedef vector<pll> vpll;
typedef array<ll,3> all3;
typedef array<ll,5> all5;
typedef vector<all3> vall3;
typedef vector<all5> vall5;
typedef vector<ll> vll;
typedef vector<vll> vvll;
typedef vector<int> vi;
typedef vector<string> vs;
typedef vector<vs> vvs;
typedef unordered_set<ll> usll;
typedef unordered_set<pll, PairHash> uspll;
typedef unordered_map<ll, ll> umll;

// macros
#define rep(i,n) for(int i=0;i<n;i++)
#define rrep(i,m,n) for(int i=n;i>=m;i--)
#define all(a) begin(a), end(a)
#define rall(a) rbegin(a), rend(a)

ll __gcd(ll x, ll y) { return !y ? x : __gcd(y, x % y); }

using namespace std;
ll solve(ll n) {
    ll k = sqrt(n), res = 0;
    for(ll i = 1; i <= k; i++) {
        res += ((n / i) - (n / (i + 1))) * i;
    }
    for(ll i = 1; i <= n / (k + 1); i++) {
        res += (n / i);
    }
    return res;
}

int main() {
    Code By Sumfi

    //cout.precision(12);

    ll tc = 1;
    //cin>>tc;
    for (ll i = 1; i <= tc; i++) {
        ll n;
        cin>>n;
        cout<<solve(n)<<endl;
    }
    return 0;
}