tests were weak + alot of people in problem B were hacked due to a simple mistake since the diffrence between the previous sum and current element in array d should be >= 0 and d[i] != 0 then ans = -1 because basically if this happened you can use in this index in answer two diffrent elements which will make the answer not unique but most did it > 0 so basically case 2 6 8 for example they will fail because 2+6 = 8 and 8-8 = 0 and ans should be equal -1. i used this test to hack 14 people.