题解 | 校门外的树

# 区间合并

L , M = map(int,input().split())

a = []
for i in range(0,M):
    l , r = map(int,input().split())
    a.append([l,r])

a.sort()

l = a[0][0]
r = a[0][1]
ans = L+1
for i in range(1,M):
    if a[i][0] > r:
        ans -= r-l+1
        l = a[i][0]
        r = a[i][1]
    elif a[i][0] <= r:
        r = max(r , a[i][1])
ans -= r-l+1
print(ans)