How to pass integers to tcl procedure in vmd

I am new in tcl programming and I need to write a script for vmd that calculates two distances between two couples of atoms and print them in an output file. I do not understand why measure can not take atom_1, etc. This is my script and thank you for your help

proc distance {distance_1 atom_1 atom_2 distance_2 atom_3 atom_4 output} {    set outfile [open $output w]    puts $outfile "frame, $distance_1, $distance_2"    set nf [molinfo top get numframes]    for {set i 0} {$i < $nf} {incr i} {            set d1 [measure bond {$atom_1 $atom_2} frame $i]            set d2 [measure bond {$atom_3 $atom_4} frame $i]            puts $outfile "$i , $d1 , $d2"    }    close $outfile    }