题解 | 校门外的树

L, M = map(int, input().split())
intervals = []
for _ in range(M):
    a, b = map(int, input().split())
    intervals.append((a, b))

intervals.sort()
merged = []
for a, b in intervals:
    if not merged:
        merged.append([a, b])
    else:
        la, lb = merged[-1]
        if a <= lb + 1:
            merged[-1][1] = max(lb, b)
        else:
            merged.append([a, b])

cut = 0
for a, b in merged:
    cut += b - a + 1

print((L + 1) - cut)