For the 2013 completion schedule, what is the total S.no.?
CREATE TABLE "hydro_power_stations_under_construction" (
  "s_no" real,
  "power_plant" text,
  "state" text,
  "total_capacity_mw" real,
  "completion_schedule" text
);
SELECT SUM("s_no") FROM "hydro_power_stations_under_construction" WHERE "completion_schedule"='2013';