#include <bits/stdc++.h>
using namespace std;
#define _for(i, l, r, vals...) \
  for (decltype(l + r) i = (l), i##end = (r), ##vals; i <= i##end; ++i)
template <class T>
bool chkmin(T &a, T b) {
  return b < a ? a = b, true : false;
}
template <class T>
bool chkmax(T &a, T b) {
  return a < b ? a = b, true : false;
}
const uint32_t OFFSET = 5;
const uint32_t N = 2e5 + OFFSET, M = 2e5 + OFFSET, K = 21;
int a[N];
auto solve() -> void {
  int n;
  cin >> n;
  _for(i, 0, n - 1) cin >> a[i];
  int ans = -1;
  _for(i, 0, n - 1)
    if (a[i] != i) ans &= i;
  cout << ans << '\n';
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int _t;
  cin >> _t;
  while (_t--) solve();
  return 0;
}