//Fahmi_Nafi_46

#include<bits/stdc++.h>
using namespace std;

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

    int count = 0;
    int odd = 0;

    for(int i = 0; i < n; i++) {
        if(x[i] % 2 == 0) {
            count += 1;
        } else {
            odd += 1;
            if(odd % 2 == 0) {
                count += 1;
            }
        }
    }
    if(odd % 2 != 0) {
        cout << -1 << endl;
    } else {
        cout << count << endl;
    }
    return 0;
}