题解 | 校门外的树

L,M = map(int,input().split())
lis = [1 for _ in range(L+1)]
for i in range(M):
    a,b = map(int,input().split())
    if 1 in lis[a:b+1]:
        lis[a:b+1] = [0] *(b-a+1)
print(lis.count(1))