Is my HDD failing as SMART parameter (Reallocated Sector count) is very high(390K)

I have a 2 year old 500GB HGST 5400rpm laptop HDD. I started tracking its SMART parameters a while ago, and number of Reallocated sectors(in decimal) has increased from 65k in Nov,2015 to 390k in May,2016.Refer to the link below to view the history of the this parameter.

History of Reallocated sectors

My HDD is working fine but I am really worried that it may one day unexpectedly fail due to increasing number of Reallocated sectors.Should I be worried?