reference failed: exit status 2: panic: runtime error: index out of range [4] with length 4

goroutine 1 [running]:
main.drop(0x0?, 0x0?)
	/home/ubuntu/codeforces/1000-1999/1500-1599/1570-1579/1575/1575J.go:17 +0x1b4
main.drop(0x0, 0x3)
	/home/ubuntu/codeforces/1000-1999/1500-1599/1570-1579/1575/1575J.go:24 +0x124
main.main()
	/home/ubuntu/codeforces/1000-1999/1500-1599/1570-1579/1575/1575J.go:53 +0x3d4

exit status 1