//
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define endl '\n'
const int maxn = 105;
int raw[maxn];
signed main(){
	ios::sync_with_stdio(0);
	cin.tie(0);
	int t;
	cin >> t;
	while(t--){
		int n,k;
		cin >> n >> k;
		for(int i = 1;i <= n;i++)cin >> raw[i];
		if(k >= 2e4){
			cout << -1 << endl;
			continue;
		}
		bool fall = 0;
		int ans = 0;
		for(int i = 1;i <= k;i++){
			for(int j = 1;j <= n;j++){
				if(j == n){
					fall = 1;
					break;
				}
				else if(raw[j] >= raw[j+1])continue;
				else{
					raw[j]++;
					ans = j;
					break;
				}
			}
			if(fall)break;
		}
		if(fall)ans = -1;
		cout << ans << endl;
	}
	return 0;
}
