Create subtraction with multiple equal values

<input type="number" name="as[0][qnt1]" class="form-control valid" aria-invalid="false">
<input type="number" name="as[0][qnt2]" class="form-control valid" aria-invalid="false">
<input type="number" name="as[0][tot]" class="form-control valid" aria-invalid="false" disabled>

<input type="number" name="as[1][qnt1]" class="form-control valid" aria-invalid="false">
<input type="number" name="as[1][qnt2]" class="form-control valid" aria-invalid="false">
<input type="number" name="as[1][tot]" class="form-control valid" aria-invalid="false" disabled>

ecc

Question:

how can I go about creating a subtraction between the two fields?