#include <bits/stdc++.h>
    using namespace std;
    typedef long long ll;
    #define pb push_back 
    #define sz(x) int(x.size())
    #define vl vector<long long> 
    #define vi vector<int>
    #define vb vector<bool>
  
    unordered_map<string,int> memo;
  
    //vector<bool> primes(1000002,1);
      
    /*void eratos(ll n)
    {
        primes[1] = 0;  
        for(ll i = 2; i <= n;i++)
        {
            if(primes[i])
            {
                for(ll j = i * i; j <= n; j += i)
                    primes[j] = 0;
            }
        }
    }*/
    int main()
    {
        ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
        ll t = 1;
        cin>>t;
        vl bul(10e5+10);
        while(t--)
        {
        	ll n,k,index = 0,mx = 0;
        	cin>>n>>k;
        	ll k1 = k;
        	vl a(n+1);
        	
        	for(int i = 1; i <= n; i++)
        	{
        		cin>>a[i];
        	}
        	for(int i = 1; i < 10e5; i++)
        	{
        		bool st = 0;
        		for(int j = 1; j <= n; j++)
        		{
        			if(j == n)
        			{
        				st = 1;
        				bul[i] = j;
        				index = i;
        				break;
        			}
        			if(a[j] < a[j+1])
        			{
        				a[j]++;
        				bul[i] = j;
        				break;
        			}
        		}
        		if(st)
        		{
        			break;
        		}
        	}
        	
        	if(index <= k)
        		cout<<-1;
        	else
        	{
        		cout<<bul[k];
        	}
        	cout<<'\n';
        	
        	

        	


        }
    }