program integration_of_gr

      real*8 ::  pi, pi4, dr, N
      real*8 :: r(300), gr(300)
      integer:: i

      open(1,file="fort.120",status='unknown')

      dr  = 0.01
      pi  = 4.0d0*datan(1.0d0)
      pi4 = 4.0d0*pi
      N   = 0.0

      do i = 1,1052
      read(1,*)r(i),gr(i)
      N  = N + pi4*r(i)*gr(i)*dr
      end do
      write(*,*)"N_Particle =",N

      stop
      end