text: #include <algorithm>
  #include <bitset>
  #include <cassert>
  #include <cctype>
  #include <cfloat>
  #include <climits>
  #include <cmath>
  #include <cstdio>
  #include <deque>
  #include <iomanip>
  #include <iostream>
  #include <list>
  #include <map>
  #include <queue>
  #include <random>
  #include <set>
  #include <stack>
  #include <string>
  #include <unordered_set>
  #include <vector>
  using namespace std;
  typedef long double ld;
  typedef long long int ll;
  typedef unsigned long long int ull;
  typedef vector<int> vi;
  typedef vector<char> vc;
  typedef vector<bool> vb;
  typedef vector<double> vd;
  typedef vector<string> vs;
  typedef vector<ll> vll;
  typedef vector<pair<int, int>> vpii;
  typedef vector<pair<ll, ll>> vpll;
  typedef vector<vi> vvi;
  typedef vector<vvi> vvvi;
  typedef vector<vc> vvc;
  typedef vector<vs> vvs;
  typedef vector<vll> vvll;
  typedef map<int, int> mii;
  typedef set<int> si;
  #define rep(i, n) for (ll i = 0; i < (n); ++i)
  #define rrep(i, n) for (int i = 1; i <= (n); ++i)
  #define arep(i, v) for (auto i : v)
  #define irep(it, stl) for (auto it = stl.begin(); it != stl.end(); it++)
  #define drep(i, n) for (int i = (n)-1; i >= 0; --i)
  #define fin(ans) cout << (ans) << '\n'
  #define STLL(s) strtoll(s.c_str(), NULL, 10)
  #define mp(p, q) make_pair(p, q)
  #define pb(n) push_back(n)
  #define all(a) a.begin(), a.end()
  #define rall(a) a.rbegin(), a.rend()
  #define Sort(a) sort(a.begin(), a.end())
  #define Rort(a) sort(a.rbegin(), a.rend())
  #define MATHPI acos(-1)
  #define fi first
  #define se second
  // #include <atcoder/all>
  // using namespace atcoder;
  constexpr int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
  constexpr int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};
  template <class T> inline bool chmax(T &a, T b) {
    if (a < b) {
      a = b;
      return 1;
    }
    return 0;
  }
  template <class T> inline bool chmin(T &a, T b) {
    if (a > b) {
      a = b;
      return 1;
    }
    return 0;
  }
  template <class T> inline void dump(T &v) {
    irep(i, v) { cout << (*i) << ((i == --v.end()) ? '\n' : ' '); }
  }
  template <class T, class U> inline void dump(map<T, U> &v) {
    irep(i, v) { cout << i->first << " " << i->second << '\n'; }
  }
  inline string getline() {
    string s;
    fflush(stdin);
    getline(cin, s);
    return s;
  }
  inline void yn(const bool b) { b ? fin("yes") : fin("no"); }
  inline void Yn(const bool b) { b ? fin("Yes") : fin("No"); }
  inline void YN(const bool b) { b ? fin("YES") : fin("NO"); }
  struct io {
    io() {
      ios::sync_with_stdio(false);
      cin.tie(0);
    }
  };
  template <typename Head, typename Value>
  auto vectors(const Head &head, const Value &v) {
    return vector<Value>(head, v);
  }
  template <typename Head, typename... Tail> auto vectors(Head x, Tail... tail) {
    auto inner = vectors(tail...);
    return vector<decltype(inner)>(x, inner);
  }
  const int INF = INT_MAX;
  constexpr ll LLINF = 1LL << 61;
  constexpr ll MOD = 1000000007; // 998244353;
  constexpr ld EPS = 1e-11;
  
  /* --------------------   ここまでテンプレ   -------------------- */
  
  int main() {
    int Q;
    cin >> Q;
    while (Q--) {
      int n;
      cin >> n;
      vi v(n);
      rep(i, n) cin >> v[i];
      int odd = 0;
      int even = 0;
      rep(i, n) {
        if (v[i] % 2 == 1)
          odd++;
        else
          even++;
      }
      if (n % 2 == 0) {
        if (odd != even) {
          fin(-1);
          continue;
        }
      } else {
        // cout<<odd<<" "<<even<<endl;
        if (odd + 1 != even) {
          fin(-1);
          continue;
        }
      }
      int ans = 0;
      rep(i, n) {
        if (v[i] % 2 == i % 2)
          continue;
        for (int j = i + 1; j < n; j++) {
          if (v[j] % 2 == i % 2 && v[j] % 2 != j % 2) {
            ans++;
            swap(v[i], v[j]);
            break;
          }
        }
      }
      fin(ans);
    }
  }
  