Do math based on partial match to previous

Hello,

I need to subtract column I, the previous row, from column I current row and output the result into column J. The issue I have is I need this done ONLY for rows where column D partially matches the first 5 characters from the previous row. Currently I have =(I3-I2) and formatted it down the column, but I am unsure how to make it only apply when there is that partial match. Thanks.