How to reset the case number to 01 every year

On my project, I have created case number by Rule Expression which is { case 13://when the process is Internal RFS { sSequence ="RFS-" + year +"-"+ CHelper.GetNextSeqValueStr("InitialRFS").ToString().PadLeft(6,'0'); break; } So my question is " How i can reset back the case number to 01 when the new year start?" For Example: If my last case number ends on 199 on 2016 but when 2017 start the case number should reset to 01.