Arccos(-1/2)+arcsin(-1/2)-arcctg(-1)

Arccos(-1/2) + arcsin(-1/2) - arcctg(-1) = 
= pi - arccos(1/2) - arcsin(1/2) - pi + arcctg(1) = 
= pi - pi/3 - pi/6 - pi + pi/4 = 
= - pi/3 - pi/6 + pi/4 = 
= - 4pi/12 - 2pi/12 + 3pi/12 = 
= - 3pi/12 = - pi/4