T = int(input())
for _ in range(T):
    N = int(input())
    A = list(map(int, input().split()))

    k = -float('inf')
    for i in range(N-1):
        k = max(k, A[i]-A[i+1]+1)
    
    if k >= 0:
        print(k)
    else:
        print(-1)