Showing negative numbers as 0

Box 1: User Input

Box 2: User Input

Box 3: User Input

Box 4: User Input

Box 5: (Box 1 - Box 2)

Box 6: (Box 3 + Box 4)

Box 7: (Box 5 - Box 6)

So I'm using the following equation for Boxes 5, 6 & 7:

var v1 = getField("Box #").valueAsString;

var v2 = getField("Box #").valueAsString;

     if (v1 && v2) {

        // Perform the calculation

        event.value = +v1 - +v2;

     } else {

         event.value = "";

     }

**Note for box 6, the in the calculation is replaced with (+).

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My question: How do I write this code to produce/show "0" if the equation results in a negative number?