Twig sum row above the summerisation

You can extend twig by adding extra filters http://symfony.com/doc/current/cookbook/templating/twig_extension.html

In your case, you can use array_sum function:

public function getFilters()
{
    return array(
        new \Twig_SimpleFilter('sum', 'array_sum'),
    );
}

if you don't want to use the controller and want to do the summing in twig then try using the set command:

{# do loop first and assign whatever output you want to a variable #}
{% set sum = 0 %}    
{% set loopOutput %}               
    {% for r in a.numbers}            
       {% set sum = sum + r.number %}        
    {% endfor %}    
{% endset %}

 sum is: {{ sum }} 

{# some content.. #} 

{{ loopOutput }}

I assume the loop was in a specific place because it is intended to output something into the template, this allows you to rearrange the load order while still displaying as you want.