cacti/rrd script gathering problem

[barzog]

I created simple data input script to monitor free space on servers (df /|tail -1|awk '{print $4}') but in logs I see that data pulled in rrd is multiplied by two(i.e.
in log CMD: /usr/local/bin/rrdtool update /usr/local/www/data/cacti/rra/albatrosROOT.rrd N:5039424 but real data is 5039424/2). Whats wrong?