In macroeconomics, pi is commonly used to represent the rate of inflation. And, as in many fields, i is used as an iterator. Which means that in your math, if

In macroeconomics, pi is commonly used to represent the rate of inflation. And, as in many fields, i is used as an iterator. Which means that in your math, if you're assuming constant inflation, you need to deflate prices in year i by a factor of e^(i*pi).