連続するフレーム画像をつなげて動画として保存したい

import os                                                                                                                                                           
import cv2

num = 728
k=0
fourcc = cv2.VideoWriter_fourcc('m', 'p', '4', 'v')
video = cv2.VideoWriter('BOS.mp4',fourcc, 20.0, (1920, 1080))

while k < num:
    img = cv2.imread('pic_'+str(k)+'.png', 0)

    # can't read image, escape
    if img is None:
        print("can't read")
        break

    # add
    video.write(img)
    print(k)
    
    k += 1

print('written')

-----------------------------------------------------------------
0
1
2
・
・
・
724
725
726
727
written