input: Write a python function to calculate the total cost of a list of items given their prices and the number of items.
  assert calculate_total_cost([10, 15, 20, 25], 4) == 70
  assert calculate_total_cost([5, 10, 15], 3) == 30
  assert calculate_total_cost([100, 200, 300, 400], 2) == 300
output: def calculate_total_cost(prices, num_items):
      return sum(prices[:num_items])