#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(i=a; i<b; i++)
#define bac(i,a,b) for(i=a;i>=b;i--)
#define repf(it,x) for(auto it= x.begin(); it!=x.end(); it++)
#define repb(it,x) for(auto it= x.rbegin(); it!=x.rend(); it++)
#define reps(i,a) for(i=0; a[i]!='\0'; i++)
#define fir first
#define sec second
#define b() begin()
#define e() end()
#define all(x) (x).begin(),(x).end()
#define pb(i) push_back(i)
#define ppb() pop_back()
#define ll long long
#define mp(a,b) make_pair(a,b)
#define hell 1000000000000000000
#define M 1000000007

int main()
{
    ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
    long long int i, j,b,d, n, m,k, t=1, x=0, y,q, sum=0, maxx, ans=0, count, minn;
    cin>> t;
    
    while(t--){
        cin>> n >>k;
        ll a[n+3];
        
        rep(i,1,n+1) cin>> a[i];
        a[i]=-hell; a[i+1]=hell; a[0]=hell;
        if(k>10000) {cout<<-1 <<"\n"; continue;}
        
        i=1;
        while(k){
            while(a[i]>=a[i+1]) i++;
            a[i]++;
            i--; k--;
        }
        if(i==n) cout<< -1;
        else cout<< i+1;
        cout<<"\n";
    }
    
}