#include <bits/stdc++.h>
using namespace std;
const long long inf = 1e12;
const long long mod = 1e9 + 7;
namespace modop {
long long madd(long long a, long long b) { return (a + b) % mod; }
long long msub(long long a, long long b) {
  return (((a - b) % mod) + mod) % mod;
}
long long mmul(long long a, long long b) {
  return ((a % mod) * (b % mod)) % mod;
}
long long mpow(long long base, long long exp) {
  long long res = 1;
  while (exp) {
    if (exp % 2 == 1) {
      res = (res * base) % mod;
    }
    exp >>= 1;
    base = (base * base) % mod;
  }
  return res;
}
long long minv(long long base) { return mpow(base, mod - 2); }
long long mdiv(long long a, long long b) { return mmul(a, minv(b)); }
}  // namespace modop
using namespace modop;
const int MOD = 1e9 + 7;
typedef decay<decltype(MOD)>::type T;
struct mi {
  T val;
  explicit operator T() const { return val; }
  mi() { val = 0; }
  mi(const long long& v) {
    val = (-MOD <= v && v <= MOD) ? v : v % MOD;
    if (val < 0) val += MOD;
  }
  friend ostream& operator<<(ostream& os, const mi& a) { return os << a.val; }
  friend bool operator==(const mi& a, const mi& b) { return a.val == b.val; }
  friend bool operator!=(const mi& a, const mi& b) { return !(a == b); }
  friend bool operator<(const mi& a, const mi& b) { return a.val < b.val; }
  mi operator-() const { return mi(-val); }
  mi& operator+=(const mi& m) {
    if ((val += m.val) >= MOD) val -= MOD;
    return *this;
  }
  mi& operator-=(const mi& m) {
    if ((val -= m.val) < 0) val += MOD;
    return *this;
  }
  mi& operator*=(const mi& m) {
    val = (long long)val * m.val % MOD;
    return *this;
  }
  friend mi pow(mi a, long long p) {
    mi ans = 1;
    assert(p >= 0);
    for (; p; p /= 2, a *= a)
      if (p & 1) ans *= a;
    return ans;
  }
  friend mi inv(const mi& a) {
    assert(a != 0);
    return pow(a, MOD - 2);
  }
  mi& operator/=(const mi& m) { return (*this) *= inv(m); }
  friend mi operator+(mi a, const mi& b) { return a += b; }
  friend mi operator-(mi a, const mi& b) { return a -= b; }
  friend mi operator*(mi a, const mi& b) { return a *= b; }
  friend mi operator/(mi a, const mi& b) { return a /= b; }
};
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long t;
  cin >> t;
  while (t--) {
    long long n;
    cin >> n;
    long long arr[n];
    long long o = 0;
    long long e = 0;
    long long wrong = 0;
    for (long long i = 0; i < n; i++) {
      cin >> arr[i];
      if (arr[i] % 2) {
        o++;
      } else {
        e++;
      }
      if (arr[i] % 2 != i % 2) {
        wrong++;
      }
    }
    if (wrong == 0) {
      cout << 0 << endl;
      continue;
    }
    if (n % 2) {
      if (o != n / 2) {
        cout << -1 << endl;
      } else {
        cout << wrong / 2 << endl;
      }
    } else {
      if (o != n / 2) {
        cout << -1 << endl;
      } else {
        cout << wrong / 2 << endl;
      }
    }
  }
}
