#include<bits/stdc++.h>
#define ll long long
#define UL unsigned long long
#define FOR(i,a,b) for(auto i=(a);i<=(b);i++)
#define FORA(x,arr) for(auto &x:arr)
#define fastio ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define elif else if
#define pb push_back
#define vi vector<int>
#define pii pair<int,int>
#define vpii vector<pair<int,int>>
#define mp(a,b) make_pair(a,b)
#define mms(a) memset(a,0,sizeof(a))
using namespace std;
int main(){
	fastio
	ll t;
	cin>>t;
	while(t--){
		ll n;
		cin>>n;
		ll x;
		ll c[2]={0,0};
		FOR(i,0,n-1){
			cin>>x;
			if(i%2==x%2)
				continue;
			else
				c[x%2]+=1;
		}
		if(c[0]==c[1])
			cout<<c[0]<<endl;
		else
			cout<<-1<<endl;
	}
}