思路：
首先对所有区间进行排序，遍历区间时当出现新区间时，把上一个合并成功的区间的长度加到$ans$中，最后总的长度减去区间总的长度。

l, m = map(int, input().split())
interval = []
for i in range(m):
    interval.append(list(map(int, input().split())))
interval.sort()

ans = 0
st, ed = interval[0]
for item in interval:
    a, b = item
    if st <= a <= ed and st <= b <= ed:
        continue
    elif st <= a <= ed and b > ed:
        ed = b
    else:
        ans += ed - st + 1
        st, ed = item
ans += ed - st + 1

print(l - ans + 1)