#include<bits/stdc++.h>
using namespace std;
#define int long long
#define mod 1000000007
#define PI 3.1415926535
#define maxn 100005
#define all(x) x.begin(),x.end()
#define speedUp ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);

void _SSGJ_()
{   
    int n,k;
    cin>>n>>k;

    vector <int> v;
    for(int i=0;i<n;i++)
    {
        int a;
        cin>>a;
        v.push_back(a);
    }
    v.push_back(-1);
    while(k > 0)
    {
        k--;
        int i=0;
        while(i < n)
        {
            if(v[i] < v[i + 1])
            {
                v[i]++;
                break;
            }
            i++;
        }
        if(i < n)
        {
            if(k == 0)
                cout<<i + 1<<"\n";
        }
        else
        {
            cout<<"-1\n";
            return;
        }
    }
}


signed main()
{
    speedUp

    #ifndef ONLINE_JUDGE   
    freopen("input.txt", "r", stdin);   
    freopen("output.txt", "w", stdout);
    #endif

    int t;
    cin>>t;
    while(t--)
    _SSGJ_();
    return 0;
}