Opening different files within if statements in fortran

I'm a beginner on FORTRAN so maybe what I'm asking is trivial, but I haven't found a solution: I have 2 different files to be read, do some calculation, and then write the result in a different file that depends on the input file. I tried fisrt a simple approach:

       real*4 counts,par,deg,fl
       integer*2 i
       character*13 input_file

       WRITE(*,*) "Name of file"
       READ(*,*) input_file
       OPEN(3,FILE=input_file)
       IF (input_file .EQ. 'filename1.dat') THEN
       OPEN(4,FILE='filename1_done.dat',STATUS='NEW')
       par=3.5
       ELSE
       OPEN(4,FILE='filename2_done.dat',STATUS='NEW')
       par=2.1
       END IF
       i = 0
       do
       i = i+1
       READ(3,*,END=99) counts,deg
       fl = 10**(counts)*par
       WRITE(4,*) counts,deg,fl
       END DO
   99  CONTINUE
       CLOSE (3)
       END

My problem is that when IF returns false, it performs the calculation with the right value for par but it still calls the output file 'filename1_done.dat'. Even including the loop inside the IF statement and using different units for the output file didn't work. I feel that I'm missing something completely obvious, but can't figure out what. PS: I never learned the syntax for FORTRAN 90, but I'm trying to.