#include <bits/stdc++.h>
using namespace std;
template <typename T>
T ii() {
  T a;
  cin >> a;
  return a;
}
struct custom_hash {
  static uint64_t splitmix64(uint64_t x) {
    x += 0x9e3779b97f4a7c15;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return x ^ (x >> 31);
  }
  size_t operator()(uint64_t x) const {
    static const uint64_t FIXED_RANDOM =
        chrono::steady_clock::now().time_since_epoch().count();
    return splitmix64(x + FIXED_RANDOM);
  }
};
int main(void) {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int t;
  cin >> t;
  while (t--) {
    int n;
    cin >> n;
    vector<int> ar(n);
    for (auto &it : ar) cin >> it;
    vector<int> count(2, 0);
    for (int i = (0); i <= (n - 1); i++)
      if (i % 2 != ar[i] % 2) ++count[i % 2];
    if (count[0] != count[1]) {
      cout << (-1) << "\n";
      continue;
    }
    cout << (count[0]) << "\n";
  }
  return 0;
}
