Sign in other table for apply Sum()

[#1]

Table 1      (Define Concept)
10     -
15     -
51     +
50     +

Table 2                (Amount)
Ncon       Tot
51         $ 1,000                                       Minus amount  $2,800 + $ 3,300 + $ 600                                     =   $   6,700
51         $ 2,500                                       Add   amount   $1,000 + $ 2.500 + $  3,500 + $ 1,750 + $ 25,000  =   $ 33,750
50         $ 3,500
50         $ 1,750                                      Real Result -> $ 27,050  not $ 40,450
10         $ 2,800
10         $ 3,300
50         $25,000
15         $   600

Ncon is a field whose value is defined by a user in table 1 that can be + or - at Therefore Table 2 can't apply a Sum (Tot) as not all values ​​have the same
sign remember this definition is found in another table.  How can I load my vars in code ?

Thank for your help