就像
如果我有：
1st line (keep)
2nd line (keep)
3rd line (keep)
4rth lines (delete)
5th (del)
6th (keep)
7nth (keep)
8th lines (keep)
9th (del)
10th (del)
11th (keep)
12th (keep)
13th (keep)
14th (del)
15th (del)
等等....