1
3
5 3 4
 6
5
5 3 4 4 5
3
1 1 1
5
4 4 3 4 4
5
5 5 4 3 2
3
1 1 2
5
5 4 3 5 5
 1
3
10 1 5

1
 1
-1
2
2
3
1
 1

//   package com.company;
import java.text.DecimalFormat;
import java.util.*;
import java.io.*;
import java.lang.*;

public class Main{
    /*
     ** || ॐ श्री गणेशाय नमः ||
     ** @𝙖𝙪𝙩𝙝𝙤𝙧 𝙅𝙞𝙜𝙖𝙧_𝙉𝙖𝙞𝙣𝙪𝙟𝙞
     ** 𝙎𝙑𝙉𝙄𝙏-𝙎𝙐𝙍𝘼𝙏
     */
    public static void main(String args[]){
        PrintWriter out=new PrintWriter(System.out);
        InputReader in=new InputReader(System.in);
        TASK solver = new TASK();
        int t=1;
         t = in.nextInt();
        for(int i=0;i<t;i++)
        {
            solver.solve(in,out,i);
        }
        out.close();
    }
    static class TASK {
        static int mod = 1000000007;

        void solve(InputReader in, PrintWriter out, int testNumber) {
          int n = in.nextInt();
          int a[] = new int[n];
          for(int i=0;i<n;i++)
              a[i]=in.nextInt();

            int ans1=a[0],ans2=0;
            boolean flag=false;
          for(int i=0;i<n;i++)
          {
              if(i-1>=0 &&a[i]>a[i-1])
              {
                  ans1=a[i]+i;
                  ans2=i;
                  flag=true;
                  break;


              }
              if(i+1<n && a[i]>a[i+1])
              {
                  ans1=a[i]+i;
                  ans2=i;
                  flag=true;
                  break;
              }


          }
          for(int i=ans2;i<n;i++)
          {
              if(a[i]>ans1)
              {
                  ans1=a[i]+i-ans2;
                  ans2=i;
              }
              else if(ans1>a[i])
                  ans1++;
          }
          if(flag)
            System.out.println(ans2+1);
          else
              System.out.println(-1);

        }
    }


    static class pair{
        int x;
        int y;
        pair(int x,int y)
        {
            this.x=x;
            this.y=y;
        }

    }
    static class Maths {
        static long gcd(long a, long b) {
            if (a == 0)
                return b;
            return gcd(b % a, a);
        }

        public static long lcm(long a, long b) {
            return (a * b) / gcd(a, b);
        }

        public static long factorial(int n) {
            long fact = 1;
            for (int i = 1; i <= n; i++) {
                fact *= i;
            }
            return fact;
        }
    }



    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private SpaceCharFilter filter;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public int read() {
            if (numChars == -1)
                throw new InputMismatchException();

            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }

                if (numChars <= 0)
                    return -1;
            }
            return buf[curChar++];
        }

        public String nextLine() {
            BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
            String str = "";
            try {
                str = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }

        public int nextInt() {
            int c = read();

            while (isSpaceChar(c))
                c = read();

            int sgn = 1;

            if (c == '-') {
                sgn = -1;
                c = read();
            }

            int res = 0;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            }
            while (!isSpaceChar(c));

            return res * sgn;
        }

        public long nextLong() {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            long res = 0;

            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            }
            while (!isSpaceChar(c));
            return res * sgn;
        }

        public double nextDouble() {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            double res = 0;
            while (!isSpaceChar(c) && c != '.') {
                if (c == 'e' || c == 'E')
                    return res * Math.pow(10, nextInt());
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            }
            if (c == '.') {
                c = read();
                double m = 1;
                while (!isSpaceChar(c)) {
                    if (c == 'e' || c == 'E')
                        return res * Math.pow(10, nextInt());
                    if (c < '0' || c > '9')
                        throw new InputMismatchException();
                    m /= 10;
                    res += (c - '0') * m;
                    c = read();
                }
            }
            return res * sgn;
        }

        public String readString() {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = read();
            }
            while (!isSpaceChar(c));

            return res.toString();
        }

        public boolean isSpaceChar(int c) {
            if (filter != null)
                return filter.isSpaceChar(c);
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public String next() {
            return readString();
        }

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);
        }
    }
}



