volume

def calculate_volume(relief):
    left_max = 0
    right_max = 0
    left = 0
    right = len(relief) - 1
    volume = 0

    while left < right:
        if relief[left] > left_max:
            left_max = relief[left]
        if relief[right] > right_max:
            right_max = relief[right]
        if left_max >= right_max:
            volume += right_max - relief[right]
            right -= 1
        else :
            volume += left_max - relief[left]
            left += 1
    return volume

a = [10, 9, 8, 7, 6, 5, 4]
print(calculate_volume(a))