To summarize:

To allow only negative numbers in the expense section:

1. Select the cells where you want to allow only negative numbers.
2. Go to the Data tab in the ribbon and click on Data Validation.
3. In the Data Validation dialog box, select "Custom" from the Allow drop-down list.
4. In the Formula box, enter the following formula: =A1<0 (assuming A1 is the first cell in the selected range).
5. Click OK to close the dialog box.

To automatically convert positive numbers to negative numbers:

1. Select the cells where you want to allow both positive and negative numbers.
2. In the formula bar, enter the following formula: =IF(A1<0,A1,-A1) (assuming A1 is the first cell in the selected range).
3. Press Enter to apply the formula to the cell.
4. Copy the formula to the rest of the cells in the range.