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?
CREATE TABLE "2010_present" (
  "year" real,
  "date" text,
  "type" text,
  "president" text,
  "location" text
);
SELECT SUM("year") FROM "2010_present" WHERE "type"='informal' AND "location"='justus lipsius building, brussels' AND "date"='23 may';