#include<bits/stdc++.h>
// #include<ext/pb_ds/assoc_container.hpp>
// #include<ext/pb_ds/tree_policy.hpp>
#define int long long int
#define endl "\n"
#define pb push_back
int sum = 0;
#define arrinp(v) for(auto &i:v) cin>>i,sum+=i;
#define arrout(v) for(auto &i:v) cout<<i<<" ";
#define newline cout<<endl;
#define all(s) (s).begin(),(s).end()
#define dbg(a) cout<<a<<endl;
#define dbg2(a,b) cout<<a<<" "<<b<<endl;
using namespace std;
// using namespace __gnu_pbds;
// typedef tree<int,null_type,less<int>, rb_tree_tag,
//             tree_order_statistics_node_update> new_data_set;
//             // order_of_key(int)  k-th largest element
//             // find_by_order(int) gives us according to the order
//             // insert() to insert in the PBDS
//             // new_data_set ST; calling
signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
    int t(1);
    cin >> t;
    while (t--) {
        sum = -1;
        int n, k;
        cin >> n >> k;
        int a[n];
        for (int i = 0; i < n; i++) {
            cin >> a[i];
        }
        while (k--) {
            sum = -1;
            int flag = 0;
            for (int i = 0; i < n - 1; i++) {
                if (a[i] < a[i + 1]) {
                    sum = i + 1;
                    a[i]++;
                    break;
                }
                if (i == n - 2) {
                    flag = 1;
                }
            }
            if (flag) {
                break;
            }
        }
        dbg(sum);
    }
    return 0;
}