题解 | 校门外的树

l,m = map(int,input().strip().split())

regin = []
for _ in range(m):
    a,b = map(int,input().strip().split())
    regin.append((a,b))

totals = l +1
remove_trees = overlaps = 0

removed = [False] * (l+1)

for (a,b) in regin:
    for i in range(a,b+1):
        if not removed[i]:
            remove_trees += 1
            removed[i] = True
        else:
            overlaps += 1

remain = totals - remove_trees
print(remain)