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

for s in range(1, n + 1):
    current_max = 0
    for i in range(1, n + 1):
        d = abs(i - s)
        val = A[i-1] + d
        if val > current_max:
            current_max = val
    if current_max < min_T:
        min_T = current_max

print(min_T)