panic: runtime error: index out of range [-8]

goroutine 1 [running]:
main.computeColor({0x40000703f0, 0x3, 0x4000010080?}, 0x50)
	/home/ubuntu/codeforces/0-999/100-199/130-139/132/verifierB.go:126 +0x458
main.main()
	/home/ubuntu/codeforces/0-999/100-199/130-139/132/verifierB.go:210 +0x198
exit status 2