#include<iostream>
#include<vector>
#include<unordered_map>
#include<utility>
#include<string>
#include<cmath>
#include<algorithm>
#include<climits>
#include<set>
#include<unordered_set>
#include<stack>
#include<queue>
#include<numeric>
#include<functional>
#include<map>
#include<iomanip>
#include<math.h>

#define fori(a, b) for (int i = a; i < b; i++)
#define forj(a, b) for (int j = a; j < b; j++)
#define print(a) cout << a << " ";
#define ll long long
#define vi vector<int>
#define vb vector<bool>
#define vll vector<long long>
#define vvi vector<vector<int>>
#define vvl vector<vector<long long>>
#define pii pair<int, int>
#define all(a) a.begin(), a.end()
#define PB(n) push_back(n);
#define F first
#define S second
#define nl cout << endl;
#define yesno(b) cout << ((b) ? "YES" : "NO")

using namespace std;

const ll MOD = 1e9 + 7;
const ll INF = LLONG_MAX;

ll power(ll a, ll b, ll mod = MOD) {
    ll res = 1;
    while(b > 0) {
        if(b & 1) res = (res * a) % mod;
        a = (a * a) % mod;
        b >>= 1;
    }
    return res;
}

vb is_prime;
void seive(int n) {
    is_prime.assign(n + 1, true);
    is_prime[0] = is_prime[1] = false; 

    for (int i = 2; i * i <= n; ++i) {
        if (is_prime[i]) {
            for (int j = i * i; j <= n; j += i)
                is_prime[j] = false;
        }
    }
}


bool isPrime(ll n){
      if(n < 2) return false;
      if(n == 2 ||  n == 3) return true;
      if(n % 2 == 0 || n % 3 == 0) return false;
      for(ll i = 5 ; i*i<=n ; i++){
            if(n % i == 0 || n % (i+2) == 0){
                 return false;
            }
      }
      return true;
}

ll gcdll(ll a, ll b) { return b == 0 ? a : gcdll(b, a % b); }
ll lcmll(ll a, ll b) { return (a / gcdll(a, b)) * b; }

void solve()
{

    ll t;
    cin >> t;

    while(t--){
  
       int n;
        cin >> n;

       vll v(n);
        fori(0,n) cin >> v[i];

        ll w = 0;

        for (int i = 0; i + 1 < n; i++) {
            if (v[i] >= v[i+1]) {
                w = max(w, v[i] - v[i+1] + 1);
            }
        }

        if (w <= n) cout << w << "\n";
        else cout << -1 << "\n";

    }
}  

/*
C
AB
*/

signed main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);

    solve();
    return 0;
}