Why for D = 8  answer is 10?  Why 9 is not answer ?

indexes               :  0 1 2  3   4 5  6  7  8  9  10
prefix-sum of a[1..10]:  0 7 11 18 18 23 29 33 37 42 46
prefix-sum of b[1..10]:  0 5 8  13 14 16 19 22 29 31 31
diff                  :  0 2 3  5  4  7  10 11 8  11 15

t = 9, diff 11, t = 10 diff = 15   and minimum t which diff >8  is 9.