c  **************************************************************
       subroutine cal_dtwist(n,i,i2,dtwist)

       implicit double precision(a-h, o-z)
       parameter(maxa=500,pi=3.1415926535d0,pi2=pi*2.d0)
       double precision twist
       integer n
       common/tor_param/ dr,cg,phi0,dlk,phii,subseg
       common/euler/ alpha(maxa),beta(maxa),gama(maxa),phi(maxa)
      

       dtwist=0.d0
       if (i.lt.i2) then
           do j=i,i2
           dtwist=dtwist+phi(j)
           enddo
       else 
          do j=i,n
             dtwist=dtwist+phi(j)
          enddo 
          do j=1,i2
             dtwist=dtwist+phi(j)
          enddo 
       endif

       dtwist=dtwist/pi2
       return
       end