Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_name_77 (year INTEGER, date VARCHAR, type VARCHAR, location VARCHAR)

### Question ###
What is the sum of Year with a Type of informal, and a Location with justus lipsius building, brussels, and a Date with 23 may?

### Accurate SQL ###

SELECT SUM(year) FROM table_name_77 WHERE type = "informal" AND location = "justus lipsius building, brussels" AND date = "23 may"