codeforces - even array

#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define fast()(ios_base::sync_with_stdio(0),cin.tie(NULL));
int main()
{
    fast()
    ll t;
    cin>>t;
    while(t--)
    {
        ll n;
        cin>>n;
        ll ara[n+1],i;
        for(i=0;i<n;i++)cin>>ara[i];
        ll wo=0,we=0;
        for(i=0;i<n;i++)
        {
            if(i%2==0 && ara[i]%2!=0)we++;
            if(i%2==1 && ara[i]%2!=1)wo++;
        }
        if(we==wo)cout<<we<<endl;
        else cout<<"-1"<<endl;
    }
}