implicit none
integer(8) i,N,M
integer(8),allocatable :: x(:)
integer(8),allocatable :: y(:)
integer(8),allocatable :: z(:)
read(*,*) N
allocate (x(N))
allocate (y(N))
allocate (z(N))
do i=1,N
   read(*,*) x(i),y(i)
end do
do i=1,N
   z(i)= y(i) -x(i)
end do
M=0
do i=1,N
   M=M+1
   if (z(i)/=z(i+1)) exit
end do
if (M==N) then
    if (z(M)>0) then
        write(*,*) z(M)
    else
        write(*,*) "-1"
    end if
else
    write(*,*) "-1"
end if
end