#int(input())
#map(int, input().split())
#list(map(int, input().split()))

T = int(input())

a = []
for i in range(T):
    N = int(input())
    A = list(map(int, input().split()))

    max1 = 10 ** 18
    min1 = -1
    while max1 - min1 > 1:
        h = (max1 + min1) // 2
        u = [0] * N
        for j in range(N):
            u[j] = A[j] + j * h
        f = 1
        for j in range(N-1):
            if u[j] >= u[j+1]:
                f = 0
                break

        if f:
            max1 = h
        else:
            min1 = h

    a.append(max1)


for x in a:
    print(x)