n = int(input())
A = list(map(int, input().split()))
min_time = float('inf')

for i in range(n):
    current_max = 0
    for j in range(n):
        current_max = max(current_max, A[j] + abs(j - i))
    min_time = min(min_time, current_max)

print(min_time)