input: Write a python function to find the index of the first occurrence of a substring within a string.
  assert searchSubstring("hello world", "world") == 6
  assert searchSubstring("openai", "ai") == 4
  assert searchSubstring("abcdef", "gh") == -1
output: def searchSubstring(string, substring):
      try:
          return string.index(substring)
      except ValueError:
          return -1