~~你怎么能`l = [0] * len`呢？~~
因为树种在$0\sim l$处，总共有$l+1$项，所以`l = [0] * len`应该为`l = [0] * (len+1)`
$\textcolor{green}{\texttt{AC}}$代码：
```python
lis = list(input().split())
len = int(lis[0])
a = int(lis[1])
i = 0
l = [0] * (len+1)
u = v = 0
count = len + 1
import sys
while(i < a):
u , v = map(int , input().split())
if(u > v):
sys.exit()
while(u <= v):
if(l[u] == 0):
l[u] = 1
count -= 1
u += 1
i += 1
print(count)
```
by wanglexi @ 2023-10-31 19:32:17
@[071228xx](/user/976359)
by wanglexi @ 2023-10-31 19:32:38