integer::N
  integer,pointer::XY(:,:)
  integer,pointer::X(:),Y(:),Z(:)
  read *,N
  allocate(XY(2,N))
  allocate(Z(N))
  read *, XY
  X => XY(1,1:N)
  Y => XY(2,1:N)
  Z = Y - X

  if (Z(1).gt.0 .and. ALL(Z.eq.Z(1))) then
     print '(i0)', Z(1)
  else
     print '(i0)', -1
  end if
end program