n = int(input())
a = list(map(int, input().split()))
max_left = 0
max_right = 0
for i in range(n):
    target = i + 1
    current = a[i]
    if current > target:
        max_left = max(max_left, current - target)
    else:
        max_right = max(max_right, target - current)
print(max_left + max_right)