# Given a array of numbers representing the stock prices of a company in chronological order, 
# write a function that calculates the maximum profit you could have made 
# from buying and selling that stock once. You must buy before you can sell it.

# For example, given [9, 11, 8, 5, 7, 10], you should return 5, 
# since you could buy the stock at 5 dollars and sell it at 10 dollars.


def sum_array(lst):
