solve =: {{
	data =. ;: fread y
	mat =. (2#%:-:#data)$".>{.|:(2,~-:#data)$data
	diags =. </.mat
	k =. <.-:#diags
	'a b' =. (k{.diags);<k}.diags
	step =. {{<<./>x+each {{(_,y);y,_}}>y}}
	<./>(step/b) step step/|.a
}}

problem81 =: solve 'p081_matrix.txt'