input: Write a python function to calculate total cost of an item given its unit price and number of units.
  assert calculate_total_cost(10, 4.50) == 45.0
  assert calculate_total_cost(5, 2.99) == 14.95
  assert calculate_total_cost(100, 1.00) == 100.0
output: def calculate_total_cost(units, unit_price):
      return units * unit_price