n = int(input())
A = list(map(int, input().split()))

min_T = float('inf')

for x in range(n):
    current_max = 0
    for i in range(n):
        dist = abs(i - x)
        temp = A[i] + dist
        if temp > current_max:
            current_max = temp
    if current_max < min_T:
        min_T = current_max

print(min_T)