/*
CODED BY mamudulsakib2019
DATE:-24/03/2024 ; TIME- 17:47 PM
BANGLADESH , SYLHET
*/
#include<bits/stdc++.h>  
using namespace std;
typedef unsigned long long  ll;

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

    vector<ll> numbers(size);
    for(ll i = 0; i < size; i++){
        cin >> numbers[i];
    }
    ll evenSumSubarrays = 0, sum = 0;

    for(ll num : numbers){
        sum += num;
        if(sum % 2 == 0){
            evenSumSubarrays++;
            sum = 0;
        }
    }
    if(sum == 0){
        cout << evenSumSubarrays << endl;
    }
    else{
        cout << -1 << endl;
    }

    return 0;
}