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