Question on Round function

I happened to see these:

round(.5, 0)

[1] 0

round(1.5, 0)

[1] 2

round(2.5, 0)

[1] 2

round(3.5, 0)

[1] 4

round(4.5, 0)

[1] 4


What is the rule here?

Should not round(.5, 0) = 1, round(2.5, 0) = 3 etc?

Thanks and regards,