Fortran beginner - output file shows NaN - fortran

Question

I wrote a simple Fortran program for the three body problem using Euler-Richardson algorithm. For some reason the output files give nothing but NaN. Would the issue be solved by using subroutines or functions?
PROGRAM Threebody
IMPLICIT none
!************** Variable declarations ***************
!real(8), parameter :: G = 6.6738D-11
INTEGER :: IOSTAT, dt, t_step, io_error, i
DOUBLE PRECISION:: st, g, m0, m1, m2, force0x, force0y, force1x, force1y, force2x, force2y, n0, n1, n2, x0,v0,x1,v1,x2,v2,y0,w0,y1
DOUBLE PRECISION:: w1,y2,w2
m0 = 1.d0 ! the masses of the three bodies
m1 = 1.d0
m2 = 1.d0
g = 9.80d0 ! m/s^2 s due to gravity
t_step = 1
y0 = 0.d0
x0 = 0.d0
v0 = 0.d0
w0 = 0.d0
y1 = 0.d0
x1 = 1.d0
v1 = 2.d0
w1 = 1.118d0
y2 = 0.d0
x2 = -1.d0
v2 = -1.118d0
w2 = 0.d0
dt = 1 ! time step
OPEN(unit=5, file='out.txt', status='replace',action='write', IOSTAT=io_error)
OPEN(unit=6, file='out1.txt', status='replace',action='write', IOSTAT=io_error)
OPEN(unit=7, file='out2.txt', status='replace',action='write', IOSTAT=io_error)
! particle no.1
DO i=0,1000,t_step ! 1000 is the total time
st=i!/10.d0
n0 = sqrt(((x2 - x1)**2 + (y2 - y1)**2)**3)
force0x = (- m1*(( x0 - x1 ) / n2)) - (m2*(( x0 - x2 ) / n1))
x0 = x0 + (w0 + 0.5*force0x*st)*st
w0 = w0 + force0x*st
force0y = (- m1 * (( y0 - y1 ) / n2)) - (m2 * (( y0 - y2 ) / n1))
y0 = y0 + (v0 + 0.5*force0y*st)*st
v0 = v0 + force0y*st
WRITE(5,*) i*t_step, x0, y0, n0
END DO
! particle no.2
DO i=0,1000,dt
st=i
n1 = sqrt(((x0 - x2 )**2 + (y0 - y2)**2)**3)
force1x = (- m2 * (( x1 - x0 ) / (n0))) - m0 * (( x1 - x2 ) / n2)
x1 = x1 + (w1 + 0.5*force1x*st)*st
w1 = w1 + force1x*st
force1y = (- m2 * ( y1 - y0 ) / n0) - (m0 * (( y1 - y2 ) / n2))
y1 = y1 + (v1 + 0.5*force1y*st)*st
v1 = v1 + force1y*st
WRITE(6,*) i*t_step, x1, y1
END DO
! particle no.3
DO i=0,1000,dt
st=i
n2 = sqrt ( ( ( x1 - x0 )**2 + ( y1 - y0 )**2 )**3 )
force2x = (- m0 * (( x2 - x0 ) / n1)) - (m1 * (( x2 - x1 ) / n0))
x2 = x2 + (w2 + 0.5*force2x*st)*st
w2 = w2 + force2x*st
force2y = (- m0 * (( y2 - y0 ) / n1)) - (m1 * (( y2 - y1 ) / n0))
y2 = y2 + (v2 + 0.5*force2y*st)*st
v2 = v2 + force2y*st
WRITE(7,*) i*t_step, x2, y2, n2
END DO
CLOSE(unit=5)
CLOSE(unit=6)
CLOSE(unit=7)
END PROGRAM Threebody

Answer 1

Compiling with -Wall in gfortran gives you a hint:
f.f90:56:0: warning: ‘n1’ may be used uninitialized in this function [-Wmaybe-uninitialized]
force0x = (- m1*(( x0 - x1 ) / n2)) - (m2*(( x0 - x2 ) / n1))
n1 and n2 are not initialized so they may contain anything. In my case, 6.95e-310, causing force0x to give NaN.
To debug this, you can print the variables on screen to check when they are becoming NaN. For that, please don't use units below 10, because they may be reserved. I think that Unit 5 is standard output so by reusing it you cannot print anything on screen.