#include <iostream>
#include <vector>
#include <iomanip>
#include <limits>

//martin02

using namespace std;

int main() {
	int t = -1;
	cin >> t;

	for (int i = 0; i < t; i++) {
		long long int n, k;
		cin >> n >> k;
		vector<long long int> v(n, 0);

		for (int j = 0; j < n; j++) {
			cin >> v.at(j);
		}

		long long int pos = 0; //Position
		for (int j = 0; j < k && pos != -1; j++) {
			pos = 0;

			for (int z = 1; z < n; z++) {
				if (v.at(z) > v.at(z - 1)) {
					pos = z - 1;
					break;
				}

				if (z == n - 1) {
					pos = -1;
					break;
				}

			}

			if (pos != -1) {
				v.at(pos) += 1;
			}

		}

		if (pos != -1 && n != 1) {
			cout << setprecision(numeric_limits<long long int>::digits10) << pos + 1 << endl;
		}
		else {
			cout << -1 << endl;
		}

	}

	return 0;
}