a=input()+' '
for i in range(len(a)):
    if a[i]==" ":
        cd=int(a[0:i])   #cd马路长度
        qy=int(a[i+1:-1])  #区域数目
        break
q2=[[0 for i in range(2)] for j in range(qy)]  #q2储存起始点终止点
for j in range(qy):
    b=input()+' '
    for k in range(len(b)):
        if b[k]==" ":
            qd=int(b[0:k])
            zd=int(b[k+1:-1])
            q2[j][0]=qd
            q2[j][1]=zd
            break
for i in range(1,len(q2)): #感觉是这后面出了问题
    for j in range(i,len(q2)):
        if q2[j][1]>=q2[i-1][0] and q2[j][0]<q2[i-1][0]:
            q2[i-1][0]=q2[j][0]
            q2[j][1],q2[j][0]=-1,-1
        elif q2[j][0]<=q2[i-1][1] and q2[j][1]>q2[i-1][1]:
            q2[i-1][1]=q2[j][1]
            q2[j][1],q2[j][0]=-1,-1
for i in range(len(q2)):
    if q2[i][0]!=-1:
        cd-=q2[i][1]-q2[i][0]+1
cd+=1
print(cd)

希望有大佬帮忙看看