how to copy value of a sharepoint column to another column in same list

I have a form which contains a column "DGM" of type "people". There is another column "Approver" which I need to be auto populated with the same value as DGM contains. I tried using the formula =[DGM] in calculated value of Approver column. But i am getting following error. I dont have share point designer so have to accomplish the task through UI. One or more column references are not allowed, because the columns are defined as a data type that is not supported in formulas. Any Help would be appreciated.