Negative Balance to be brought forward

In performing a calculation in Forms, how can someone insert a negative value in a numerical text field?
The form on this link https://sdf.org.jm/qr, asks the user to give a report each quarter (calculation involved) and some users need to insert a negative balance brought forward from the previous period. However the numerical field does not allow for that negative insert.

How can I include the negatives brought forward in the field?

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