#include<bits/stdc++.h>
using namespace std;
vector<int>ps(vector<int>&arr){
    vector<int>pref(arr.size()+1,0);
    for(int i=1;i<=arr.size();i++){
    	pref[i]=pref[i-1]+arr[i-1];
    }
    return pref;
}
int main(){
    vector<int>arr={2,4,6,8};
    vector<int>pref=ps(arr);
    cout<<pref[3]-pref[0]<<"\n";  // sum[1,3] = 12
    return 0;
}