l = [[4, 3, 2, 1], [5, 6, 7, 8], [12, 11, 10, 9], [13, 14, 15, 16]]
print(sum([x if i&1 else x[::-1] for i,x in enumerate(l)],[]))
bCA9IFtbNCwgMywgMiwgMV0sIFs1LCA2LCA3LCA4XSwgWzEyLCAxMSwgMTAsIDldLCBbMTMsIDE0LCAxNSwgMTZdXQpwcmludChzdW0oW3ggaWYgaSYxIGVsc2UgeFs6Oi0xXSBmb3IgaSx4IGluIGVudW1lcmF0ZShsKV0sW10pKQ==
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]