/*
problem :
algorithm :
created : Programmer_juruo
*/

#include <bits/stdc++.h>
using namespace std;

#define int long long
#define N 2000005
#define M 2005
#define lowbit(x) x & (-x)
#define F1(i, n) for(int i = 1; i <= n; i++)
#define F(i, a, n) for(int i = a; i <= n; i++)
#define F0(i, n) for(int i = 0; i < n; i++)
#define dbg(x) cout << #x << ":" << x << endl;
#define se second
#define fi first
#define y1 juruo
#define mp make_pair
#define pb push_back
#define arr vector<int>
#define chmin(a, b) a = min(a, b)
#define chmax(a, b) a = max(a, b)

typedef pair <int, int> pii;
typedef priority_queue <int> pq;
typedef long long ll;

const int inf = 0x3f3f3f3f;
const int p = 107;
const int P = 100007;
const int Mod = 1e9+7;

int n, m;
int a[N];

void work() {
	cin >> n >> m;
	for(int i = 1; i <= n; i++) {
		cin >> a[i];
	} 
	for(int i = 1; i <= m; i++) {
		int j; 
		for(j = 1; j < n; ) {
			if(a[j] >= a[j+1]) j++;
			else {
				++a[j];
				break;
			}
		}
		if(j == n) {
			cout << -1 << endl;
			return ; 
		}
		if(i == m) {
			cout << j << endl;
		}
	} 
}

signed main() {
  //freopen("1.in", "r", stdin);
  //freopen("1.out", "w", stdout);
  int T = 1;

  cin >> T;

  while(T--) {
  	work();
  }

  return 0;
}

