#include <iostream>
#include <vector>

using namespace std;

int main() {
    int size;
    cin >> size;

    vector<int> n(size);
    for (int i = 0; i < size; i++) {
        cin >> n[i];
    }

    int even = 0;
    int sum = 0;

    for (int i = 0; i < size; i++) {
        sum += n[i];
        if (sum % 2 == 0) {
            even++;
            sum = 0;
        }
    }

    if (sum == 0) {
        cout << even << endl;
    } else {
        cout << -1 << endl;
    }

    return 0;
}