#include <iostream>
using namespace std;

int max_even_subarray(int arr[], int n) {
    int count = 0;
    int odd_count = 0; 
    for (int i = 0; i < n; i++) {
        if (arr[i] % 2 != 0) {
            odd_count++;
            if (odd_count == 2) {
                count++;
                odd_count = 0;
            }
        } else {
            count++; 
        }
    }
    if (odd_count != 0)
        return -1;
    return count;
}

int main() {
    int n;
    cin >> n; 
    int arr[n];
    for (int i = 0; i < n; i++) {
        cin >> arr[i]; 
    }
    cout << max_even_subarray(arr, n) << endl;

    return 0;
}