In Excel parlance, the parameters you give inside a formula are called
"arguments". Since that argument is hardcoded into the formula, it
won't change as you copy it down. However, you can replace the 13 with
a cell reference to a cell that contains the number 13- that way when
you copy the formula down that cell reference will change, and you can
change the value in the cell referenced by the second formula to
contain 14.