#include<bits/stdc++.h>
using namespace std;
#define     ll              long long
#define     IOS             ios_base::sync_with_stdio(false);cin.tie(0);
#define     endl            "\n"
#define     deb(x)          cout<<#x<<" : "<<x
// #define     mod             (int)1e9+7
#define     up              upper_bound
#define     lb              lower_bound
#define     fo(i,n)         for(i=0;i<n;i++)
#define     fi              first 
#define     se              second 
#define     mp              make_pair 
#define     pb              push_back
#define     all(x)          x.begin(), x.end()
#define     clr(x)          memset(x, 0, sizeof(x))
#define     sortall(x)      sort(all(x))
#define     tr(it, a)       for(auto it = a.begin(); it != a.end(); it++)
#define     PI              3.1415926535897932384626

#define isOn                (S, j) (S & (1 << j))
#define setBit              (S, j) (S |= (1 << j))
#define clearBit            (S, j) (S &= ~(1 << j))
#define toggleBit           (S, j) (S ^= (1 << j))
#define lowBit              (S) (S & (-S))
#define setAll              (S, n) (S = (1 << n) - 1)
#define modulo              (S, N) ((S) & (N - 1))
#define isPowerOfTwo        (S) (!(S & (S - 1)))
#define nearestPowerOfTwo   (S) ((int)pow(2.0, (int)((log((double)S) / log(2.0)) + 0.5)))
#define turnOffLastBit      (S) ((S) & (S - 1))
#define turnOnLastZero      (S) ((S) | (S + 1))
#define turnOffLastConsecutiveBits       (S) ((S) & (S + 1))
#define turnOnLastConsecutiveZeroes      (S) ((S) | (S - 1))


typedef pair<ll, ll>    pii;
typedef pair<ll, ll>    pl;
typedef vector<ll>      vi;
typedef vector<ll>      vl;
typedef vector<pii>     vpii;
typedef vector<pl>      vpl;
typedef vector<vi>      vvi;
typedef vector<vl>      vvl;



#define TRACE
#ifdef TRACE
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) {
    cout << name << " : " << arg1 << endl;
    //use cerr if u want to display at the bottom
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
    const char* comma = strchr(names + 1, ','); cout.write(names, comma - names) << " : " << arg1 << " | "; __f(comma + 1, args...);
}
#else
#define trace(...)
#endif

ll mod = 1000000007;




int main(){
#ifndef ONLINE_JUDGE
    freopen("input1.txt","r",stdin);
    freopen("output1.txt","w",stdout);
#endif
    IOS;
    ll t=1;
    cin>>t;
    while(t--)
    {
        ll n;ll k;cin>>n>>k;
        ll a[n];
        for(ll i = 0;i<n;i++)   
        {
            cin>>a[i];
        }
        int flag = 0;ll ans = 0;
        for(ll i = 0;i<k;i++)
        {
            flag = 0;
            for(ll j = 0;j<n-1;)
            {
                if((a[j] - a[j+1])>=0)
                {
                    j++;
                }
                else
                {
                    a[j]++;
                    ans = j+1;
                    flag = 1;
                    break;
                }
            }
            if(flag == 0)
            {
                break;
            }

        }
        if(flag == 0)
        {
            cout<<-1<<endl;
        }
        else
        {
            cout<<ans<<endl;
        }

        

    }

    return 0;
        
        
}
