substr() function on number values do not return correct values in webi 3.1

Hi friends,

I have a 6 digit number value like 122012 which denotes month Dec and Year 2012 .I have to seperate the month as 12 and Year as 2012. I defined a variable using below formula,it gave me invalid datatype

=(Substr([SMM Realized Savings Month Code];1;4))

How can I convert the value to string before applying SUBSTR.

I didnt find tochar function in the list

Thanks

Reena