l,m=map(int,input('').split())

a=[]

b=[]

c=[]

for i in range(l):

c.append('*')

for i in range(m):

a.append('*')
b.append('*')

for i in range(m):

a[i],b[i]=map(int,input().split())

for i in range(len(a)):

for f in range(a[i],b[i]):
    c[f]='p'

h=0

for i in range(len(c)):

if c[i]=='*':
     h+=1

print(h-1)