#include <bits/stdc++.h>
using namespace std;
#define ll long long int
const int N = 1e7 + 9;
int a[N];

int32_t main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    int n;
    cin >> n;
    for (int i = 1; i <= n; i++)
    {
        cin >> a[i];
    }
    ll sum = 0;
    ll cnt = 0;
    for (int i = 1; i <= n; i++)
    {
        sum = max(1LL * a[i], a[i] + sum);
        if (sum % 2 == 0)
        {
            cnt++;
            sum = 0;
        }
    }
    if (cnt == 0)
    {
        cout << "-1\n";
    }
    else
    {
        cout << cnt << '\n';
    }

    return 0;
}