Recursion for finding the smallest sum of numbers in an array

Question

I have a problem.

I need to sum up the arrays values to get the lowest possible sum.

Rules:

You go to next step and take the value (step 1->2)

You skip 2 and take double the value (step 1->4, skipping the 2nd and 3rd elements)

in an array of values 1 5 1 60 1 5 3

for this array the answers would be 20 but the code gives the result 30

So far I have partial solution:

static int recur(int [] arr, int index, int sum)
    {
        if (index >= arr.Length) return 1000;
        sum += arr[index];
        if (index == arr.Length - 1)
        {
            return sum;
        }
        else
        {
            return Math.Min(recur(arr, index + 1, sum), 2* recur(arr, index + 3, sum));
        }
    }