How to add summary formula in Reports in Salesforce?

Steps of add summary formula in the report:
1. create a report
2. From the Groups section, select a field from the Add group... lookup menu under GROUP ROWS.
3. From the Columns section, click Show More | Add Summary Formula Column.
4. Enter a name for the summary formula column.
5. Choose the Formula Output Type.
6. Enter a summary formula.
7. To see if your formula contains errors, click Check Syntax. Resolve any errors.
8. Click Apply.
9. Click Save.