题解 | 校门外的树

L,M=map(int,input().split())
dict={x:True for x in range(L+1)}

for i in range(M):
    x,y=map(int,input().split())
    for j in range(x,y+1):
        dict[j]=False
res=0
for key in dict:
    if dict[key]==True:
        res+=1
print(res)