def head(arr):
    return arr[0] if len(arr) > 0 else None

def last(arr):
    return arr[len(arr) - 1] if len(arr) > 0 else None

def init(arr):
    return arr[:-1]

def tail(arr):
    return arr[1:]

def take(n, arr):
    return arr[:n]

def drop(n, arr):
    return arr[n:]

def reverse(arr):
    return list(reversed(arr))

############

def byDefault(default, value):
    return default if value is None else value

def fromIndex(i, arr):
    try:
        return arr[i]
    except:
        return None

############

def spiral(matrix):
    if not matrix:
        return []

    left_column = []
    right_column = []
    new_matrix = []
    for row in init(tail(matrix)): # mid rows
        left_column.append(head(row))
        right_column.append(last(row))
        new_matrix.append(init(tail(row)))

    return byDefault([], head(matrix)) + \
           reverse(right_column) + \
           reverse(byDefault([], last(tail(matrix)))) + \
           left_column + \
           spiral(new_matrix)

def operands(arr):
    if not arr:
        return 0

    if len(arr) == 1:
        return head(arr)

    [a, b] = take(2, arr)

    adds = a + b
    subs = adds - byDefault(0, fromIndex(2, arr))
    mult = subs * byDefault(1, fromIndex(3, arr))

    return operands([mult] + drop(4, arr))

############

matrix = [
    [5,4,7],
    [1,2,3],
    [3,2,1],
]

print(operands(spiral(matrix)) == 14)