panic: runtime error: index out of range [21] with length 20

goroutine 1 [running]:
main.genCase(0x40000a4e80)
	/home/ubuntu/codeforces/1000-1999/1400-1499/1490-1499/1492/verifierC.go:58 +0x2d0
main.main()
	/home/ubuntu/codeforces/1000-1999/1400-1499/1490-1499/1492/verifierC.go:81 +0x25c
exit status 2