CF1637C Andrew and Stones

Andrew has $ n $ piles with stones. The $ i $ -th pile contains $ a_i $ stones. He wants to make his table clean so he decided to put every stone either to the $ 1 $ -st or the $ n $ -th pile. Andrew can perform the following operation any number of times: choose $ 3 $ indices $ 1 \le i < j < k \le n $ , such that the $ j $ -th pile contains at least $ 2 $ stones, then he takes $ 2 $ stones from the pile $ j $ and puts one stone into pile $ i $ and one stone into pile $ k $ . Tell Andrew what is the minimum number of operations needed to move all the stones to piles $ 1 $ and $ n $ , or determine if it's impossible.

The input contains several test cases. The first line contains one integer $ t $ ( $ 1 \leq t \leq 10\,000 $ ) — the number of test cases. The first line for each test case contains one integer $ n $ ( $ 3 \leq n \leq 10^5 $ ) — the length of the array. The second line contains a sequence of integers $ a_1, a_2, \ldots, a_n $ ( $ 1 \leq a_i \leq 10^9 $ ) — the array elements. It is guaranteed that the sum of the values $ n $ over all test cases does not exceed $ 10^5 $ .

For each test case print the minimum number of operations needed to move stones to piles $ 1 $ and $ n $ , or print $ -1 $ if it's impossible.

In the first test case, it is optimal to do the following: 1. Select $ (i, j, k) = (1, 2, 5) $ . The array becomes equal to $ [2, 0, 2, 3, 7] $ . 2. Select $ (i, j, k) = (1, 3, 4) $ . The array becomes equal to $ [3, 0, 0, 4, 7] $ . 3. Twice select $ (i, j, k) = (1, 4, 5) $ . The array becomes equal to $ [5, 0, 0, 0, 9] $ . This array satisfy the statement, because every stone is moved to piles $ 1 $ and $ 5 $ . There are $ 4 $ operations in total.In the second test case, it's impossible to put all stones into piles with numbers $ 1 $ and $ 3 $ : 1. At the beginning there's only one possible operation with $ (i, j, k) = (1, 2, 3) $ . The array becomes equal to $ [2, 1, 2] $ . 2. Now there is no possible operation and the array doesn't satisfy the statement, so the answer is $ -1 $ . In the third test case, it's optimal to do the following: 1. Select $ (i, j, k) = (1, 2, 3) $ . The array becomes equal to $ [2, 0, 2] $ . This array satisfies the statement, because every stone is moved to piles $ 1 $ and $ 3 $ . The is $ 1 $ operation in total.In the fourth test case, it's impossible to do any operation, and the array doesn't satisfy the statement, so the answer is $ -1 $ .