#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define fast ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define endl '\n'



int main()
{
    fast;
    ll t=1;
    cin>>t;

    while(t--)
    {
        ll n, k;
        cin>>n>>k;

        ll h[n];
        for(int i=0;i<n;i++)
            cin>>h[i];

        ll cnt = 0;
        ll i=0;
        ll ans = 0;

        while( (cnt < k) && (i<n-1) )
        {
            if(h[i]>=h[i+1]) i++;
            else
            {
                ans = i+1;
                cnt++;

                h[i]++;
                if(i>0 && h[i-1]<h[i]) i--;
            }
        }

        if( (cnt<k) || i==n-1)
            cout << -1 << endl;
        else
            cout<< ans << endl;
    }

    return 0;
}
