panic: runtime error: index out of range [3] with length 3

goroutine 1 [running]:
main.(*Fenwick).Sum(...)
	/home/ubuntu/codeforces/1000-1999/1900-1999/1920-1929/1928/1928F.go:29
main.contributionB(0x40000a0d58?)
	/home/ubuntu/codeforces/1000-1999/1900-1999/1920-1929/1928/1928F.go:178 +0x1e8
main.addSegmentB(0x4)
	/home/ubuntu/codeforces/1000-1999/1900-1999/1920-1929/1928/1928F.go:218 +0x90
main.main()
	/home/ubuntu/codeforces/1000-1999/1900-1999/1920-1929/1928/1928F.go:326 +0xa44
exit status 2
reference error: exit status 1
exit status 1