Untitled

#include 
#include 
#include 
#include 

using namespace std;
typedef long long ll;
int main()
{
    int n;
    cin >> n;
    vector v(n);
    map> mapa;
    ll sum = 0, result = 0;
    for(int i = 0; i < n; i++) {
        cin >> v[i];
        sum += v[i];
        mapa[sum].push_back(i);
    }
    sum = 0;
    for(int i = n - 1; i >= 0; i--) {
        sum += v[i];
        if(mapa.count(sum)) {
            if(mapa[sum][0] < i) {
                result = max(result, sum);
            }
        }
    }
    cout << result << endl;
        return 0;
}