def main():
    import sys
    input = sys.stdin.read().split()
    N = int(input[0])
    A = list(map(int, input[1:N+1]))
    
    min_time = float('inf')
    
    for s in range(1, N + 1):
        left_max = 0
        for i in range(1, s + 1):
            temp = A[i - 1] + (s - i)
            if temp > left_max:
                left_max = temp
        
        right_max = 0
        for i in range(s, N + 1):
            temp = A[i - 1] + (i - s)
            if temp > right_max:
                right_max = temp
        
        current_max = max(left_max, right_max)
        if current_max < min_time:
            min_time = current_max
    
    print(min_time)

if __name__ == "__main__":
    main()