CREATE TABLE table_name_77 (year INTEGER, date VARCHAR(255), type VARCHAR(255), location VARCHAR(255))
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?
    
SQL: SELECT SUM(year) FROM table_name_77 WHERE type = "informal" AND location = "justus lipsius building, brussels" AND date = "23 may"