Why do I get List index out of range when trying to add consecutive numbers in a list using for i in list

Question

Given the following list

a = [0, 1, 2, 3]

I want to create a new list b, which consists of elements for which the current and next values of a are summed. It will contain 1 less element than a.

Like this:

b = [1, 3, 5]

Here's what I've tried:

b = []

for i in a:

    b.append(a[i + 1] + a[i])

The trouble is I keep getting this error:

IndexError: list index out of range

Can somone help me with this?