#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define gcd(a,b) (__gcd(a,b))
#define lcm(a,b) ((a*b)/gcd(a,b))
vector<ll> soe(ll n) {ll *arr = new ll[n + 1]; vector<ll>ans; for (ll i = 0; i <= n; i++) {arr[i] = 1;} arr[0] = 0; arr[1] = 0; for (ll i = 2; i <= sqrt(n); i++) {if (arr[i] == 1) {for (ll j = 2; i * j <= n; j++) {arr[i * j] = 0;}}} for (ll i = 0; i <= n; i++) {if (arr[i] == 1)ans.push_back(i);} return ans;}

const ll mod = 1e9 + 7;
/* ncr = n-1cr-1+ n-1cr  */

/******************************************************************/

void solve() {
	ll n, k;
	cin >> n >> k;
	ll a[n];
	for (int i = 0; i < n; i++)
		cin >> a[i];

	ll temp = k;
	int pos = -1, flag = 1, rr = 0;
	while (flag == 1 and k > 0) {
		int i = 0;
		flag = 0;
		while (i < n - 1) {
			if (a[i] < a[i + 1]) {
				a[i] += 1;
				k--;
				rr++;
				flag = 1;
				pos = i + 1;
				break;
			}
			i++;
		}
	}
	// flag = 0;
	// for (int i = 0; i < n; i++) {
	// 	if (a[i] < a[i + 1])
	// 		flag = 1;
	// }
	if (rr ==  temp )
		cout << pos << endl;
	else
		cout << -1 << endl;
}

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
#ifndef ONLINE_JUDGE
	freopen("inputf.in", "r", stdin);
	freopen("outputf.in", "w", stdout);
#endif
	int test;
	cin >> test;
	while (test--) {
		solve();
	}
}