SUBROUTINE revcst(x,y,iorder,ncity,n,de)
      INTEGER ncity,iorder(ncity),n(6)
      REAL de,x(ncity),y(ncity)
      INTEGER ii,j
      REAL alen,xx(4),yy(4),x1,x2,y1,y2
      alen(x1,x2,y1,y2)=sqrt((x2-x1)**2+(y2-y1)**2)
      n(3)=1+mod((n(1)+ncity-2),ncity)
      n(4)=1+mod(n(2),ncity)
      do 11 j=1,4
        ii=iorder(n(j))
        xx(j)=x(ii)
        yy(j)=y(ii)
11    continue
      de=-alen(xx(1),xx(3),yy(1),yy(3))-alen(xx(2),xx(4),yy(2),yy(4))+
     *alen(xx(1),xx(4),yy(1),yy(4))+alen(xx(2),xx(3),yy(2),yy(3))
      return
      END
C  (C) Copr. 1986-92 Numerical Recipes Software .