<think>
Okay, I need to implement the take_back function. Let's see the problem statement again. The function takes a list of values and a target integer. The goal is to take entries from the end of the list towards the start and determine how many must be taken before their total reaches the target. If the whole list never reaches the target, return -1.

Hmm, so the function should sum elements starting from the end of the list, adding them one by one, and check when the sum meets or exceeds the target. The return value is the number of elements taken. If adding all elements doesn't reach the target, return -1.

Wait, but what if the target is zero? The problem says that the target is an integer, but it's not clear if the target could be zero. However, the function needs to handle that. But the problem says to return -1 if the whole list never reaches it. So if the target is zero, and the sum of all elements is zero, then it's possible. But if the target is zero and the sum of all elements is not zero, then maybe it's not possible. But the problem says to return -1 when the whole list never reaches the target. So, for example, if the target is zero, and the sum of all elements is zero, then the number of elements taken would be the length of the list. But if the target is zero and the sum is not zero, then maybe it's impossible. Wait, but the target is zero. So if the sum of all elements is zero, then the total is zero. But if the sum is not zero, then even after taking all elements, the total is not zero. So in that case, return -1.

But how to approach this? Let's think of the steps.

First, iterate over the list starting from the end. For each element, add it to a cumulative sum. Keep track of how many elements have been added. Once the cumulative sum is >= target, return the count. If after adding all elements, the sum is still less than the target, return -1.

But wait, the problem says "how many must be taken before their total reaches a target". So the sum of the taken elements must reach the target. So, for example, if the target is 10, and the first element (from the end) is 5, then adding that gives 5, which is less than 10. Then the next element is 6, adding to 11, which is >=10. So the count is 2.

But if the target is 10, and the elements are [5, 6], then taking the first element (6) gives 6 <10, then adding 5 gives 11 >=10. So the count is 2.

So the approach is to accumulate the sum starting from the end, and check after each addition if the sum meets or exceeds the target. If at any point it does, return the count. If after all elements, it's still not met, return -1.

But what if the target is zero? Let's say the list is [3, -3]. The target is zero. Taking the first element (from the end) is -3. Sum is -3 <0. Then take the next element, 3. Sum is 0, which meets the target. So the count is 2.

Another example: target is 0, list is [1, 1]. The sum of all elements is 2, which is not zero. So even after taking all elements, the sum is 2, which is not zero. So return -1.

So the function needs to process