#include <bits/stdc++.h>

#define endl '\n'
#define fi first
#define se second
#define pb push_back
#define _ ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);

using namespace std;

typedef long long lli;
typedef unsigned long long ulli;
typedef pair<lli, lli> pii;

///-----------------------------------------------------------
///-----------------------------------------------------------



///-----------------------------------------------------------



int main()
{   _

    int t;
    cin >> t;
    while (t--)
    {
        int n, k;
        cin >> n >> k;
        vector<int> h(n + 2);
        for (int i=1; i<=n; ++i)
            cin >> h[i];
        h[n + 1] = -1;
        bool sale = false;
        int flecha;
        while (k--)
        {
            flecha = 1;
            while (flecha <= n && h[flecha] >= h[flecha + 1])
                ++flecha;
            if (flecha > n)
            {
                sale = true;
                break;
            }
            else
                ++h[flecha];
        }
        if (sale)
            cout << "-1" << endl;
        else
            cout << flecha << endl;
    }

    return 0;
}
