#include <bits/stdc++.h>
typedef long long ll;
typedef unsigned long long ull;
using namespace std;
int main() {
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    int ttt;
    cin >> ttt;
    while (ttt--) {
        int n, k;
        cin >> n >> k;
        vector<int> a(n);
        for (int i = 0; i < n; i++) {
            cin >> a[i];
        }
        for (int j = 0; j < k; j++) {
            int id = -2;
            for (int i = 0; i < n - 1; i++) {
                if (a[i] < a[i + 1]) {
                    a[i]++;
                    id = i;
                    break;
                }
            }
            if (j == k - 1 || id < 0) {
                cout << id + 1 << endl;
                break;
            }
        }
    }
    return 0;
}