题解 | 校门外的树

from re import T
import sys
L,M = map(int,input().split())
#种树区域设为1
TreeArea = [1]*(L+1)
for _ in range(M):
    #施工区域树设为0
    startArea,endArea = map(int,input().split())
    for i in range(startArea,endArea+1):
        TreeArea[i]=0
    
print(sum(TreeArea))