program trapezio
      implicit double precision (a-h,o-z) 
      parameter (nd=1000)
      dimension x(nd),y(nd)

      open(1,file='points.dat')
      do i=1,nd
            read(1,*,end=10)x(i),y(i)
      enddo
      stop  'Increase dimension (x.y)'
  10  n=i-1
      close(1)

      call trap(n,x,y,sum,nd)
      write(*,*)' Sum = ', sum
 
      end


c *********************************************************************
      subroutine trap(n,x,y,sum,nd)
c *********************************************************************
      implicit double precision (a-h,o-z)  
      dimension x(nd),y(nd)

      sum=0.d0
      do i=2,n
        if(x(i).le.x(i-1)) stop ' x NOT in INCREASING order '
        sum=sum+.5d0*(x(i)-x(i-1))*(y(i)+y(i-1))
      enddo
      
      return
      end