Segment tree (sum) // build, update & query

#include <bits/stdc++.h>

/** sorce for code: https://www.hackerearth.com/practice/data-structures/advanced-data-structures/segment-trees/tutorial/ **/

 /********* Credits *********/
 /// Coded by:             ///
 ///     - NikolaP         ///
 ///                       ///
 /// Compiler: C++ 14      ///
 /// Date: 28 - Aug - 2019 ///
 /***************************/

/** typedefs & #defines **/
//{

#define fi first
#define se second
#define pb push_back
#define mp make_pair

#define forn(i,n)    for(int i = 0 ; i < (int)(n) ; ++i)
#define forv(i,l,n)  for(int i = (int)(l) ; i < (int)(n) ; ++i)

#define all(x)     (x).begin(), (x).end()
#define allr(x)    (x).rbegin(), (x).rend()
#define mems(a,x)  memset((a),(int)(x),sizeof((a)))
#define memsb(a,x) memset((a),(bool)(x),sizeof(a))

using namespace std;

typedef long long int ll;
typedef double ld;
typedef pair<int,int> ii;
typedef vector<int> vi;
typedef vector<ii> vii;
typedef vector<ll> vll;
typedef vector<vi> vvi;
typedef vector<vll> vvll;

//}



int tree[4002]; // size = n_max * 4
int arr[1000];  // size = n_max

void build(int node, int start, int end){
    if(start == end)
        tree[node] = arr[start];

    else{
        int mid = (start + end) / 2;

        build(2*node, start , mid);
        build(2*node + 1, mid+1, end);

        tree[node] = tree[2*node] + tree[2*node + 1];
    }
}

void update(int node, int start, int end, int id, int val){
    if(start == end){
        arr[id] += val;
        tree[node] += val;
    }

    else{
        int mid = (start + end) / 2;

        if(start <= id and id <= mid)
            update(2*node, start , mid , id , val);

        else
            update(2*node + 1, mid+1 , end , id , val);

        tree[node] = tree[2*node] + tree[2*node +1];
    }
}

int query(int node, int start, int end, int l, int r){
    if(r < start or end < l)
        return 0;

    if(l <= start and end <= r)
        return tree[node];

    int mid = (start + end) / 2;
    int p1 = query(2*node, start , mid, l , r);
    int p2 = query(2*node+1, mid+1 , end, l , r);
    return (p1 + p2);
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.precision(8);
    // cout << fixed;

    int n;  cin >> n;

    forn(i,n){
        cin >> arr[i];
    }

    build(1,0,n-1);

    cout << query(1,0,n-1,1,2);
    return 0;
}