I want to add the least six numbers

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CrystalReports

Please I want to add students grades in each subject together to get the total grade. I will like to add only the least six numbers out of the total numbers which are more than six.
I assign numbers to the score which represent the individual grades.

Below is an example of what I mean:

SUBJECT  Score  GRADE
 
Math      80     1
English   60     3
French    70     2
social    42     5
Science   60     3
ICT       80     1
Pre-Tech  60     3
Pre-Voc   50     4

So I want to add six of the grades starting from the least so I can get the best six.
In this case the answer will be 1+1+2+3+3+3= 13
Best six grade= 13

What I have tried:

Tonumber(sum(minimum({@grade}),6))