code: def read_png_depth(filename: str) -> int | None:
      """Read the special tEXt chunk indicating the depth from a PNG file."""
      with open(filename, "rb") as f:
          f.seek(-(LEN_IEND + LEN_DEPTH), 2)
          depthchunk = f.read(LEN_DEPTH)
          if not depthchunk.startswith(
              DEPTH_CHUNK_LEN + DEPTH_CHUNK_START
          ):
              # either not a PNG file or not containing the depth chunk
              return None
          else:
              return struct.unpack("!i", depthchunk[14:18])[0]
  
imports: from __future__ import annotations
  import binascii
  import struct
label_desc: Use image node_id 1 for calling a global function with example usage: read_png_depth(filename) and returns: None, struct