$x = 0 = sum fom. 0 tom. k=0
$x = 1 = sum fom. 0 tom. k=1
$x = 3 = sum fom. 0 tom. k=2
$x = 6 = sum fom. 0 tom. k=3
$x = 10 = sum fom. 0 tom. k=4
$x = 15 = sum fom. 0 tom. k=5
$x = 21 = sum fom. 0 tom. k=6
$x = 28 = sum fom. 0 tom. k=7
$x = 36 = sum fom. 0 tom. k=8
$x = 45 = sum fom. 0 tom. k=9
$x = 55 = sum fom. 0 tom. k=10