#include <bits/stdc++.h>
#define ll long long
#define F first
#define S second
#define endl '\n'
#define Endl '\n'

using namespace std;

const int N = 2e5 + 5;
int tc, n, m, a[N];

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0); // cout.tie(0);
    cin >> n;
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    long long sum = accumulate(a, a + n, 0ll);
    if (sum & 1) {
        cout << -1 << endl;
    } else {
        int cnt = 0;
        sum = 0;
        for (int i = 0; i < n; i++) {
            sum += a[i];
            if (sum % 2 == 0) {
                cnt++;
                sum = 0;
            }
        }
        cout << cnt << endl;
    }

    return 0;
}