What is the 2nd leg if the 1st leg is 3-1 and the Team 2 is union sportive de la médina d'alger?
CREATE TABLE "round_16_matches" (
  "team_1" text,
  "agg" text,
  "team_2" text,
  "1st_leg" text,
  "2nd_leg" text
);
SELECT "2nd_leg" FROM "round_16_matches" WHERE "1st_leg"='3-1' AND "team_2"='union sportive de la médina d''alger';