Jimmy wants to test the performance of two different pairs of shoeson his running times.

Each time he runs, he flips a coin to determine which pair of shoes he's going to wear and then measures the time (in minutes) it takes him to run 10km.

He ran wearing Pair 1 on 15 occasions (mean of 42:03, standard deviation of 3:16) and Pair 2 on 15 occasions (mean of 40:29, standard deviation of 4:23).

What is the value for the degrees of freedom (using n₁ + n₂ -2)?