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

# Compute L: L[i] is the maximum time when reaching i-th cell (1-based) from left
L = [0] * (n + 1)
for i in range(1, n + 1):
    L[i] = max(L[i-1] + 1, A[i-1])

# Compute R: R[i] is the maximum time when reaching i-th cell (1-based) from right
R = [0] * (n + 2)
for i in range(n, 0, -1):
    R[i] = max(R[i+1] + 1, A[i-1])

min_time = float('inf')
for i in range(1, n + 1):
    left = L[i-1]
    right = R[i+1]
    current = max(left, right) + 1
    current = max(current, A[i-1])
    if current < min_time:
        min_time = current

print(min_time)