def img_av(img, x, y)
  if x >= 0 && x < img[0].length() && y >= 0 && y < img.length() 

    width = img[0].length()
    height = img.length()

    # For ignoring invalid sub-scriptions
    leftEnd0 = 1 - (width - x) / width
    rightEnd0 = 1 - (x + 1) / width
    top0 = 1 - (height - y) / height
    bottom0 = 1 - (y + 1) / height

    # Use modulo in order to avoid accessing invalid areas
    sum = img[y][x] +
      img[y][(x-1) % width] * leftEnd0 + img[y][(x+1) % width] * rightEnd0 +
      img[(y-1) % height][x] * top0 + img[(y+1) % height][x] * bottom0 +
      img[(y-1) % height][(x-1) % width] * leftEnd0*top0 +
      img[(y-1) % height][(x+1) % width] * rightEnd0*top0 +
      img[(y+1) % height][(x-1) % width] * leftEnd0*bottom0 +
      img[(y+1) % height][(x+1) % width] * rightEnd0*bottom0
  
    length = 1 + leftEnd0 + rightEnd0 + top0 + bottom0 +
      leftEnd0 * top0 + rightEnd0 * top0 + leftEnd0 * bottom0 + rightEnd0 * bottom0
  
    sum * 1.0 / length
  else
    -1
  end
end