#include <bits/stdc++.h>
#include <unordered_map>

using namespace std;

int main() {
    int n;
    cin >> n;
    unordered_map<int, int> _map;
    for (int i = 0; i < n; i++) {
        int c;
        cin >> c;
        if (c > 0) {
            _map[c] = 1;
        } else {
            if (_map.count(abs(c)) > 0) {
                _map[abs(c)]++;
            }
        }
    }
    //遍历map，找到值为2的
    int count = 0;
    unordered_map<int, int>::iterator it = _map.begin();
    while (it != _map.end()) {
        if (it->second == 2) count++;
        it++;
    }
    cout << count << endl;
    return 0;
}