有大佬知道哪里有问题吗，只能通过一半测试点

l, m = map(int, input().split())
areas = []

for i in range(m):
    u, v = map(int, input().split())
    j = 0
    while j < len(areas):
        # [150, 300] [100 ,200] 左交集
        if areas[j][0] >= u and areas[j][0] <= v and areas[j][1] >= v:
            areas[j][0] = u
            break
        # [100, 200] [150, 300] 右交集
        elif areas[j][0] <= u and areas[j][1] >= u and areas[j][1] <= v:
            areas[j][1] = v
            break
        # [100, 300] [150, 160] 包含
        elif areas[j][0] <= u and areas[j][1] >= v:
            break
        # [150, 160] [100, 300] 被包含
        elif areas[j][0] >= u and areas[j][1] <= v :
            del areas[j]
            j -= 1
        j += 1

    if(len(areas) == 0): # 首次加入
        areas.append([u, v])
    elif j >= len(areas): # 非首次, 遍历完后无交集, 加入
        areas.append([u, v])

remove_area = 0

for area in areas:
    remove_area += (area[1] - area[0] + 1)

print(l - remove_area + 1)