Maximum Sum Subarray problem

Write a function called max_sum that takes v, a row vector of numbers, and n, a positive integer as inputs. The function needs to find the n consecutive elements of v whose sum is the largest possible. In other words, if v is [1 2 3 4 5 4 3 2 1] and n is 3, it will find 4 5 and 4 because their sum of 13 is the largest of any 3 consecutive elements of v. If multiple such sequences exist in v, max_sum returns the first one. The function returns summa, the sum as the first output argument and index, the index of the first element of the n consecutive ones as the second output. If the input n is larger than the number of elements of v, the function returns 0 as the sum and -1 as the index.

Here are a few example runs:

[summa, index] = max_sum([1 2 3 4 5 4 3 2 1],3)
    summa = 13 
    index = 4
[summa, index] = max_sum([1 2 3 4 5 4 3 2 1],2) 
    summa = 9 
    index = 4
[summa, index] = max_sum([1 2 3 4 5 4 3 2 1],1) 
    summa = 5 
    index = 5
[summa, index] = max_sum([1 2 3 4 5 4 3 2 1],9) 
    summa = 25 
    index = 1
[summa, index] = max_sum([1 2 3 4 5 4 3 2 1],10) 
    summa = 0 
    index = -1