Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
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?

### Inputs:
table_name: table, columns: ['Year', 'Date', 'Type', 'President', 'Location']

### Response:
SELECT SUM Year FROM table WHERE Type = informal AND Location = justus lipsius building, brussels AND Date = 23 may