c***Euler program from Koonin, Chapter 2
20    print *, 'Enter step size ( .le. 0 to stop)'
      read *,h
      if(h. le. 0.) stop
      nstep=3./h                   !number of steps to reach x=3
      y=1.                         !y(0)=1
      do ix=0,nstep-1              !loop over steps
         x=ix*h                    !last x value
         y=y+h*func(x,y)           !new Y value from Eq. 2.6
         diff=exp(-0.5*(x+h)**2)-y !compare with exact value
         print *, ix,x+h,y,diff
      enddo
      goto 20                      !start again with new value of h
      end
c******************************************************************
      function func(x,y)
      func=-x*y
      return
      end
c******************************************************************