ml=list(map(int,input().split()))
ml[0]=ml[0]+1
bl=[]
for i in range(ml[1]):
    tl=list(map(int,input().split()))
    bl.append(tl)
list=[]
for i in range(ml[0]):
    list.append(1)
for i in bl:
    list[i[0]]=0
    for j in range(i[1]-i[0]):
        list[j+i[0]+1]=0
t=0
for i in range(len(list)):
    if list[i]==1:
        t+=1
print(t)
/**************************************************************
	Problem: 1470
	User: admin
	Language: Python
	Result: Accepted
	Time:914 ms
	Memory:34616 kb
****************************************************************/