altitudes = [300, 450, 550, 800, 1200]

print(altitudes[0])       # first element
print(altitudes[-1])      # last element
print(altitudes[1:3])     # elements 1 up to (not including) 3
print(altitudes[:2])      # from the start through index 1
print(altitudes[2:])      # from index 2 to the end
print(altitudes[::-1])    # reversed


300
1200
[450, 550]
[300, 450]
[550, 800, 1200]
[1200, 800, 550, 450, 300]