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

#define debug(x) cout<<#x<<" = "<<x<<endl;
#define sz(a) int((a).size())
#define F(i, a, b) for(int i = a; i < (b); i++)
#define F1(i, a, b) for(int i = a; i <= (b); i++)
#define dF(i, a, b) for(int i = a; i > (b); i--)
#define dF1(i, a, b) for(int i = a; i >= (b); i--)
#define YES cout<<"yes\n"
#define NO cout<<"no\n"
typedef long long ll;
const int N = 1e6 + 10;
const int mod = 1e9 + 7;
int n, m, k;
int a[N],h[N];
char s[N];
void solve() {
	cin >> n >> k;
	F1(i, 1, n) {
		cin >> h[i];
	}
	bool ok = false;
	int ans = -1;
	F1(i, 1, k) {
		bool temp = true;
		F1(j, 1, n - 1) {
			if (h[j + 1] > h[j]) {
				ans = j;
				h[j]++;
				temp = false;
				break;
			}
		}
		if (temp) {
			ok = true;
			break;
		}
	}
	if (ok)cout << -1 << endl;
	else cout << ans << endl;
}

int main() {
	ios::sync_with_stdio(false); cin.tie(nullptr);
	int t = 1;
	cin >> t;
	while (t--) solve();
	return 0;
}