#include<bits/stdc++.h>
using namespace std;
#define ll long long
int main(){
   ll t;
   cin>>t;
   while(t--){
     ll n,k,flag=-1,out;
     cin>>n>>k;
     vector<ll> h(n);
     for(auto &x:h)
        cin>>x;
    for(ll i=1;i<=k;i++){
        int t=-2;
        for(ll j=0;j<n-1;j++){
            if(h[j]<h[j+1]){
            t=j;break;}
        }
        out=t+1;
        if(t==-2) break;
        h[t]++;
        
    }
    cout<<out<<endl;
   }  
    return 0;
}