#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 1e6;
const int MAX_N_T = 4*MAX_N;
int tree[MAX_N_T];
int nums[MAX_N];
void build(int node ,int l,int r ){
    if(l == r){
        tree[node] = nums[l];
        return;
    }else{
        int mid = (l+r)>>1;
        build(node<<1,l,mid);
        build(node<<1|1,mid+1,r);
        tree[node] = tree[node<<1] + tree[node<<1|1];
    }
}
int query(int node , int l,int r,int ql,int qr ){
    if(ql>r || qr<l)return 0;
    if(ql<=l &&  qr>= r)return tree[node];
    int mid = (l + r)>>1;
    int left = query(node<<1,l,mid,ql,qr);
    int right = query(node<<1|1,mid+1,r,ql,qr);
    return left+right;
}
void update(int node ,int l,int r, int index , int value){
    if(l == r){
        tree[node] = value;
        return;
    }
    int mid = (l + r)>>1;
    if(index<=mid){
        update(node<<1,l,mid,index,value);
    }else{
        update(node<<1|1,mid+1,r,index,value);
    }
    tree[node] = tree[node<<1] + tree[node<<1|1];
}

int main(){
    int n;
    cin>>n;
    for(int i =1 ; i <= n ; i ++){
        cin>>nums[i];
    }
	build(1,1,n);
	update(1,1,n,2,100);//索引值为2的元素更新成100
    cout<<query(1,1,n,1,4);//查询索引值1->4的元素
    
	return 0;
}