I've got the following issue: the task it pretty straightforward , i've got a sequence of images that i'd like to accumulate ( Max comp mode basically ) , so i've made one file node reading initial sequence , another reading previous frame accumulation result and finally rop node writing this frame operation to accumulated images sequence.As simple as that

http://prntscr.com/9xa0hf [prntscr.com]
Initial sequence file name pattern : $F.png
Accumulated sequence file name pattern : acc_`$F-1`.png
This frame accumulated image pattern : acc_$F.png
So it should work, i've no idea what's wrong ( but i'm sure it's my ignorance not Houdini's

Thx!
P.S : i've got the scene attached, just in case^^)