def is_sort(a, n):
    for i in range(n-1):
        if a[i] > a[i+1]:
            return False
    return True


if __name__ == '__main__':
    n = int(input())
    a = [int(i) for i in input().split()]
    if is_sort(a, n):
        print(max(a) - min(a))
    else:
        print(-1)