1
3
5 3 4
 6
5
5 3 4 4 5
3
1 1 1
5
4 4 3 4 4
5
5 5 4 3 2
3
1 1 2
5
5 4 3 5 5
 1
3
10 1 5

1
 1
-1
2
2
3
1
 1

#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 5;
const int inf = 1e9 + 7;
int main() {
  ios::sync_with_stdio(false);
  int t;
  cin >> t;
  while (t--) {
    int n;
    cin >> n;
    long long *a = new long long[n + 1];
    long long temp = 0;
    int index = 0;
    for (int i = 1; i <= n; i++) {
      cin >> a[i];
      if (a[i] > temp) {
        temp = a[i] + i - 1;
        index = i;
      } else if (a[i] < temp) {
        temp++;
      } else {
        index = i;
      }
    }
    if (temp == a[1]) {
      cout << -1 << endl;
    } else {
      cout << index << endl;
    }
  }
  return 0;
}
