DO

//#define _GLIBCXX_DEBUG
//#include <bits/stdc++.h>
//dd_definitions("-D_GLIBCXX_DEBUG")

#include <iostream>
#include <sstream>
#include <algorithm>
#include <vector>
#include <deque>
#include <queue>
#include <string>
#include <set>
#include <cmath>
#include <tuple>
#include <map>
#include <random>
#define int long long

 using namespace std;

typedef long long ll;
typedef long double ld;

const int inf = 1e9 + 7;
const int mod = 1e9 + 7;

vector <int> tree, a;

int f(int x, int y){
    return x + y;
}

int n, k, sz=1;

void build(int i = 0, int l = 0, int r = sz){
    if (r == l + 1) tree[i] = a[l];
    else{
        int m = (r + l) / 2;
        build(2 * i + 1, l, m);
        build(2 * i + 2, m, r);
        tree[i] = f(tree[2 * i + 1], tree[2 * i + 2]);
    }
}

int get(int ql, int qr, int v = 0, int vl = 0, int vr = sz){
    if(ql <= vl && vr <= qr) return tree[v];
    if(qr <= vl || vr <= ql) return 0;

    int m = (vr + vl) / 2;
    int v1 = get(ql, qr, 2* v + 1, vl, m);
    int v2 = get(ql, qr, 2* v + 2, m, vr);
    return f(v1, v2);
}

void update(int q_i, int val, int v = 0, int vl = 0, int vr = sz){
    if(q_i < vl || q_i >= vr) return;
    if(vr == vl + 1) tree[v] = val;
    else{
        int m = (vr + vl) / 2;
        update(q_i, val, 2 * v + 1, vl, m);
        update(q_i, val, 2 * v + 2, m, vr);
        tree[v] = f(tree[2 * v + 1], tree[2 * v + 2]);
    }
}

signed main(){
    cin >> n;
    a.resize(n, 0);

    while (sz < n){
        sz = 2 * sz;
    }


    a.resize(sz, 0);
    tree.resize(2 * sz - 1, 0);

    for (int i = 0; i < n ;i++){
        cin >> a[i];
    }

    build();
    cout << get(1, 3) <<'\n';
    update(2, 100);
    cout << get(2, 3) <<'\n';