#include <bits/stdc++.h>
using namespace std;
int main()
{
  long long N,A[300000],count = 0,m = 0;
  cin >> N;
  for(int i = 0;i<N;i++)
  {
    cin >> a[i];
  }
  for(int i = 0;i<N;i++)
  {
    for(int k = i+1;k<N;k++)
    {
      m = a[i]+a[k];
      if(m == 0){count++;}
    }
  }
  cout << m << endl;
  return 0;
}