[`ไอ้`เกย์เก็ต]

(a+1)+(b+2)+(c+3)+(d+4) = 4(a+b+c+d+5)
(a+b+c+d)+10 = 4(a+b+c+d+5)
3(a+b+c+d) = 10-20 = -10

ดังนั้น 3(a+b+c+d) = -10 #