#include<bits/stdc++.h>
#define FASTIO ios_base::sync_with_stdio(false); cin.tie(NULL);cout.tie(NULL);
#define mod 1000000007
#define INF 10000000000000001
#define F first
#define S second
#define LB lower_bound
#define UB upper_bound
#define vc vector
#define vll vector<long long>
#define pll pair<long long,long long>
#define pb push_back
#define all(v) v.begin(),v.end()
#define T ll test;cin>>test; while(test--)
#define forn(i,a,n) for(ll i=a;i<(long long)n;++i)
#define backn(i,n,a) for(ll i=n;i>(long long)a;--i)
#define MAX 200005
#define sz(x) (ll)x.size()
#define ins insert
#define mp make_pair
#define lld  long long
#define cy cout<<"Yes\n";
#define cno cout<<"No\n";
typedef  long long ll;
typedef long double ld;
using namespace std;
void solve()
{
}
signed main()
{
  	FASTIO
  	T
  	{
  		
	ll n;
	ll k;
	cin >> n >> k;
	ll h[n];
	forn(i,0,n)
		cin>>h[i];
		
	int large=*max_element(h,h+n);
	if(n*large<k)
	{
		cout << "-1\n";
		continue;
	}
	ll d=0;
	for(ll i=0;i<k;i++)
	{
		int t=-2;
		for(int j=0;j<n-1;j++)
		{
			if(h[j+1]>h[j])
			{
				t=j;
				break;	
			}
		}
		d=t+1;
		if(t==-2)
		{
			break;
		}
		h[t]++;
	}
		
	cout << d <<endl;
	}
}