#include <iostream>
#include <string.h>
#include <bits/stdc++.h>
#pragma GCC target("avx2")
#pragma GCC optimization("O3")
#pragma GCC optimization("unroll-loops")
using namespace std;
typedef vector<int> vi;
typedef pair<int, int> pii;
#define endl "\n"
#define sd(val) scanf("%d", &val)
#define ss(val) scanf("%s", &val)
#define sl(val) scanf("%lld", &val)
#define debug(val) printf("check%d\n", val)
#define all(v) v.begin(), v.end()
#define PB push_back
#define MP make_pair
#define FF first
#define SS second
#define ll long long
#define MOD 1000000007
#define clr(val) memset(val, 0, sizeof(val))
#define what_is(x) cerr << #x << " is " << x << endl;
#define OJ                            \
    freopen("input.txt", "r", stdin); \
    freopen("output.txt", "w", stdout);
#define FIO                           \
    ios_base::sync_with_stdio(false); \
    cin.tie(NULL);                    \
    cout.tie(NULL);
int main()
{
    int t, n, k;
    cin >> t;
    while (t--)
    {
        int w = 1, f = 0;
        cin >> n >> k;
        int a[n];
        for (int i = 0; i < n; i++)
        {
            cin >> a[i];
        }
        while (k--)
        {
            int flag = 0, x = 1;
            for (int i = 0; i < n - 1; i++)
            {

                if (a[i] < a[i + 1])
                {
                    a[i]++;
                    break;
                }
                x++;
                if (x == n)
                {
                    break;
                }
            }
            if (x == n)
            {
                flag = 1;
                if (flag == 1)
                {
                    cout << -1 << endl;
                    break;
                }
            }

            else
            {
                if (k == 0)
                {
                    w = x;
                    f = 1;
                }
            }

            // else{
            //     cout<<
            // }

            // for (auto &x : a)
            // {
            //     cout << x << " ";
            // }
            // cout << endl;
        }
        if (f == 1)
        {
            cout << w  << endl;
        }
    }

    return 0;
}
