input: Write a function to find the position of a substring within a string and return -1 if not found.
  assert find_substring_position("Python is powerful & fun", "pow") == 10
  assert find_substring_position("Python is powerful & fun", "Python") == 0
  assert find_substring_position("Python is powerful & fun", "fun") == 20
  assert find_substring_position("Python is powerful & fun", "notfound") == -1
output: def find_substring_position(main_string, substring):
      return main_string.find(substring)