def get_number(question, condition, error):
    number = 0
    while True:
        try:
            number = int(raw_input(question + "\n"))
        except ValueError:
            print
            print 'Input must be numerical.'
            print
            continue

        if condition(number):
            break
        else:
            print
            print error
            print

    return number

# Get the amount of ingredients.
amount = get_number('How many ingredients?', \
                    lambda x: x > 0, \
                    'Amount must be greater than 0.')

# Get the ingredient info and the total cost.
total_cost = 0
for i in xrange(1, amount + 1):
    prefix = '#' + str(i) + ': '

    stored = get_number(prefix + 'How many do you have?', \
                        lambda x: x >= 0, \
                        'Surely you cannot have a negative amount.')
    needed = get_number(prefix + 'How many do you need?', \
                        lambda x: x >= 0, \
                        "You have more somewhere, I'm sure of it.")
    cost   = get_number(prefix + 'How much does one cost?', \
                        lambda x: x > 0, \
                        "That's a deal too good to be true!")

    if stored < needed:
        total_cost += cost * (needed - stored)

# Print the result.
print "Total cost: " + str(total_cost)