input: Write a function to identify the largest number in the given list. If the list is empty, return -1.
  assert find_largest([1000, 506, 299, 620, 35])==1000
  assert find_largest([-1, -5, -10, -2])==-1
  assert find_largest([42])==42
  assert find_largest([])==-1
output: def find_largest(lst):
      if not lst:
          return -1
      return max(lst)