subroutine dittdrein(p,l1,l2,costhdd)
      implicit none
      include 'constants.f'
      integer j,l1,l2
      double precision costhdd,p(mxpart,4),pt,ptem,ptep,xl
      double precision pem(4),pep(4),psum(4)
      double precision p_cm(4)

      do j=1,4
      pem(j)=p(l1,j)
      pep(j)=p(l2,j)
      psum(j)=pem(j)+pep(j)
      enddo

      ptem=pt(l1,p)
      ptep=pt(l2,p)


      if (ptem .gt. ptep) then
      call boosta(psum,pem,p_cm)
      else
      call boosta(psum,pep,p_cm)
      endif

      xl=sqrt((psum(1)**2+psum(2)**2+psum(3)**2)
     .       *(p_cm(1)**2+p_cm(2)**2+p_cm(3)**2))
      costhdd=(p_cm(1)*psum(1)+p_cm(2)*psum(2)+p_cm(3)*psum(3))/xl

      return
      end