Not understanding why this won't sum up properly
<pre><code>grades = [100, 100, 90, 40, 80, 100, 85, 70, 90, 65, 90, 85, 50.5]

def grades_sum(grades):
    sum = 0
    for i in grades:
        sum += grades[i]
print(grades_sum(grades))
</code></pre>

<p>That's my code and I'm trying to understand why I'm getting an out of index traceback.</p>
