#include <bits/stdc++.h>
using namespace std;
typedef unsigned long long int lu;
typedef long long int ll;
typedef long double ld;
typedef vector<ll> v;
typedef vector<v> vv;
typedef vector<ld> vld;
typedef vector<bool> vb;
typedef vector<string> vs;
typedef pair<ll, ll> pll;
typedef vector<set<ll>> vsll;
typedef set<pair<ll, ll>> spll;
typedef vector<spll> vspll;
typedef vector<pll> vpll;
const ll MOD = 1e9 + 7;
const v dx = {1, -1, 0, 0};
const v dy = {0, 0, 1, -1};
#define round(x, y) ((2 * x + y) / (2 * y))
#define ce(x, y) ((x + y - 1) / y)
#define lcm(x, y) ((x) * (y) / __gcd(x, y))
#define sz(x) (ll) x.size()
#define sq(x) ((x) * (x))
 
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
 
void solve() {

    ll n,k;
    cin >>n>>k;

    vector<ll> data(n);

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

    ll cnt=0;
    while(true)
    {

        ll boulder=-1;
        for(int i=0;i<n-1;i++)
        {
            if(data[i]<data[i+1])
            {
                data[i]+=1;
                boulder=i+1;
                break;
            }
        }

        cnt+=1;
        if(cnt==k)
        {
            cout << boulder <<"\n";
            return;
        }
    
        if(boulder==-1)
        {
            cout <<-1<<"\n";
            return;
        }
    }

}
 
int main(void) {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    ll t;
    cin >>t;
    while(t--)
    solve();
    return 0;
}

// 10 9 6 2 8

// 10 9 6 2 8