P = (1,5) and Q = (11,10) are points on the elliptic curve y² = x³ + x over the field Z₂₃.

P + Q = R = (x_R , y_R ) where s = (y_P - y_Q) / (x_P - x_Q) (mod p) = (-5)/(-10) = 2^-1 = 12 and

x_R = s² - x_P - x_Q (mod p) = 144-1-11 = 17 and y_R = -y_P + s(x_P - x_R) (mod p) = -5 + 12(-16) = 10.

So P + Q =(17,10)

I would like to know how (mod p) is found in this instance