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

for i in range(n):
    current_max = 0
    for j in range(n):
        dist = abs(i - j)
        val = A[j] + dist
        if val > current_max:
            current_max = val
    if current_max < min_total:
        min_total = current_max

print(min_total)