Find number betwen commans

I have this file:
[p2,115,0,4,] Steel Drums
[p2,116,0,4,] Woodblock
[p2,117,0,4,] Taiko
[p2,119,10,4,] Sine Perc.
[p2,118,0,4,] Melo. Tom 1
[p2,118,18,4,] Jungle SDHow to subtract “1” to the first value between comans
,115,
,116,
,117,
,118,
,118,
etcThanks

You would have to script a solution to that.
A similar discussion that would show you how to do it is HERE.In your case your “find” expression could be:
p2,(\d+),
and you could replace with'p2,' + str(int(m.group(1))  1) + ','
or something close to that; there are a few different ways to write these expressions.