// Code by Prashubh Atri
#include <bits/stdc++.h>
using namespace std;

void solve(){
    int n,k,arr[101],i;
    // memset(arr, 0, 101);
    cin>>n>>k;
    for(i=0;i<n;i++){cin>>arr[i];}
    int pos = 0;
    while(k--){
        i=0;
        while(i<n-1 && arr[i]>=arr[i+1]){
            i++;
        }
        if(i==n-1){
            pos=-1;
            break;
        }else{
            pos = i + 1;
            arr[pos-1]++;
        }
    }
    cout<<pos;
    return;
}

int main(){
    int t;cin>>t;while(t--){
        solve();
        cout<<'\n';
    }
    return 0;
}