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

try:
    for _ in range(int(input())):
        n = int(input())
        arr = [int(x)+(n-1) for x in input().split()]
        
        if len(set(arr))==1:
            print("-1")
        else:
            a = max(arr)
            ans = arr.index(a)
            for i in range(ans,n-1):
                if arr[i]>arr[i+1]:
                    ans = i
                    break

            print(ans+1)
            


except :
    pass