//on the name of Allah:)
#include<bits/stdc++.h>
#define int         long long
#define Endl        "\n"
#define pi          2 * acos(0.0)
#define mod         1000000007
#define Mul(a,b)    (a%mod * b%mod)%mod
#define Add(a,b)    (a%mod + b%mod)%mod
#define all(x)      (x).begin(),(x).end()
#define allr(x)     (x).rbegin(),(x).rend()
#define gcd(x, y)   (__gcd(x, y))
#define lcm(x, y)   ((x/gcd(x, y))*y)
#define faster      cin.tie(NULL), cout.tie(NULL);
#define TC          int t ; cin>>t ; for(int i=1;i<=t;i++)
const int N = 1e6 + 7;
using namespace std;
void s()
{
    int n;
    cin>>n;
    vector<int>v(n);
    for(int i=0; i<n; i++)cin>>v[i];

    int cnt = 0;
    int sum= 0;
    int sumall=0;
    for (auto it : v)
    {
        sum += it;
        sumall += it;
        if (sum % 2 == 0)
        {
            cnt++;
            sum = 0;
        }
    }
    if(sumall%2!=0)cout<<"-1"<<endl;
    else cout<<cnt<<endl;
}
int32_t main()
{
    ios::sync_with_stdio(false);
    //faster
    //TC
    {
        //cout<<"Case "<<i<<": ";
        s();

    }



}