program main
   implicit none
   integer:: n,m,i,ans=0
   integer,allocatable,dimension(:)::l,r,a
   read(*,*)n,m
   allocate(l(m),r(m),a(n))
   do i=1,m
      read(*,*)l(i),r(i)
   enddo
   a=0
   do i=1,m
     a(l(i):r(i))=a(l(i):r(i))+1  
   enddo
   do i=1,n
     if(a(i)==m)ans=ans+1
   enddo
   write(*,*)ans
   
   
 
 
 
 
 
 
 
   
contains
  subroutine heapsort(n,array)
  implicit none
  integer,intent(in) :: n
  integer,intent(inout) :: array(1:n)
  
  integer ::i,k,j,l
  integer :: t
  
  if(n.le.0)then
     write(6,*)"Error, at heapsort"; stop
  endif
  if(n.eq.1)return

  l=n/2+1
  k=n
  do while(k.ne.1)
     if(l.gt.1)then
        l=l-1
        t=array(L)
     else
        t=array(k)
        array(k)=array(1)
        k=k-1
        if(k.eq.1) then
           array(1)=t
           exit
        endif
     endif
     i=l
     j=l+l
     do while(j.le.k)
        if(j.lt.k)then
           if(array(j).lt.array(j+1))j=j+1
        endif
        if (t.lt.array(j))then
           array(i)=array(j)
           i=j
           j=j+j
        else
           j=k+1
        endif
     enddo
     array(i)=t
  enddo

  return
end subroutine heapsort
end program main
