panic: runtime error: index out of range [-1]

goroutine 1 [running]:
main.refSolve({0x400010a000, 0x78})
	/tmp/cf-worker-2178506665/verifierA.go:164 +0xf54
main.main()
	/tmp/cf-worker-2178506665/verifierA.go:273 +0x1ac