The following table shows the number of elected persons from various parties in a state assembly. 

Party/years	2015	2016	2017	2018	2019	2020
P	62	70	52	90	82	80
Q	80	90	60	70	76	70
R	90	80	100	50	80	65
S	55	60	70	80	75	72

Find the difference between the number of seats won by party R and party Q in all the elections put together.