proc myCOMcalc {N masseven massodd} {
   set totmass 0.
   set comx 0.
   set comy 0.
   for {set i 1} {$i<=$N} {incr i} {
      for {set j 1} {$j<=$N} {incr j} {
         set prodij [expr {$i*$j}]
         if {[expr {$prodij % 2}]==0} {
            set comx [expr {$comx+$masseven*double($i)}]
            set comy [expr {$comy+$masseven*double($j)}]
            set totmass [expr $totmass+$masseven]
         } else {
            set comx [expr {$comx+$massodd*double($i)}]
            set comy [expr {$comy+$massodd*double($j)}]
            set totmass [expr $totmass+$massodd]
         }
      }
   }
   return "[expr {$comx/$totmass}] [expr {$comy/$totmass}]"
}

puts [myCOMcalc 10 2. 1.]