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

goroutine 1 [running]:
main.solve.func1(...)
	/tmp/cf-worker-1721869668/verifierF.go:94
main.solve({0x400009a000, 0xf7})
	/tmp/cf-worker-1721869668/verifierF.go:156 +0xdb8
main.main()
	/tmp/cf-worker-1721869668/verifierF.go:32 +0xe8