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

int main() {
    int n;
    cin >> n ;

    int arr[n+1];
    arr[0] ={0};
    for (int i = 1; i < n+1; ++i) {
        cin >> arr[i];
    }
    
    int even = 0;
    for(int i=1; i<=n; i++)
    {
        arr[i] = arr[i] + arr[i-1];
        if(arr[i]%2==0){
            even++;
        }
    }
    if(even >0){
        cout<<even<<endl;
    }
    else{
        cout<<-1<<endl;
    }

    return 0;
}