input: Below is an instruction that describes a task. Write a response that appropriately completes the request.
  
  ### Instruction:
  Write a function to determine if a substring exists in a given string. If it exists, return the starting index of the substring. If it does not exist, return -1.
  
  ### Input:
  
  
  ### Output:
  def find_substring(source, substring):
      '''This function returns the start index of a substring in given string '''
      if substring not in source:
          return -1
      i=0
      while i < len(source):
          if source[i] == substring[0]:
              flag = True
              for j in range(len(substring)):
                  if substring[j] != source[i+j]:
                      flag = False
                      break
              if flag:
                  return i
          i += 1
      return -1
  
  assert find_substring("hello world", "world")==6
  assert find_substring("hello world", "lo")==3
  assert find_substring("hello world", "or")==7
  assert find_substring("hello world", "xyz")==-1
  assert find_substring("", "")==0
output:     
      if substring not in source:
          return -1
      i=0
      while i < len(source):
          if source[i] == substring[0]:
              flag = True
              for j in range(len(substring)):
                  if substring[j] != source[i+j]:
                      flag = False
                      break
              if flag:
                  return i
          i += 1
      return -1
  