Here is you modified example (followed by screenshots of RIDE 
):
*** Settings ***
Test Template ${first} plus ${second} should equal ${expected_sum}
*** Test Cases *** FIRST SECOND RESULT
First case 1 2 3
Another case 5 5 10
Special case 3 12 15
Quite a different case
500 200 700
*** Keywords ***
${first} plus ${second} should equal ${expected_sum}
${sum} = Set Variable ${${first} + ${second}}
Should Be Equal As Integers ${sum} ${expected_sum}
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