try this:
1
1
5
1 7 13 20 27

Answer always should be >= 0. Add even more additional checks

I guess these should help

1 8
1 1
Answer: 0.00

1 8
1 3
Answer: 0.00

1 8
1 4
Answer: 2.34

1 8
1 15
Answer: 2.34

1 8
1 16
Answer: 0.00