diff --git a/backend/src/app.py b/backend/src/app.py
index cd551c2..8602d86 100644
--- a/backend/src/app.py
+++ b/backend/src/app.py
@@ -1,14 +1,42 @@
-from flask import Flask
+from flask import Flask, request, Response, jsonify
 from flask_cors import CORS
 
+from .classes import Room
+
 app = Flask(__name__)
 CORS(app)
 
 
+ROOMS: dict[int, Room] = {}  # { room_id: room, ... }
+
+
+def error(msg: str, status: int = 400) -> Response:
+    res = jsonify({"success": False, "error": msg})
+    res.status_code = status
+    return res
+
+
 @app.route("/api")
 def index():
     return "hello from flask"
 
 
+@app.route("/api/join")
+def join():
+    room_id = request.args.get("room")
+    code = request.args.get("code")
+
+    if room_id is None:
+        return error("Missing room id")
+
+    if (room := ROOMS.get(int(room_id))) is None:
+        return error("Invalid room")
+
+    if room.pin is not None and room.pin != code:
+        return error("Invalid code")
+
+    return {"success": True, "ws": f"/ws/{room_id}"}
+
+
 if __name__ == "__main__":
     app.run(debug=True)
diff --git a/backend/src/classes.py b/backend/src/classes.py
new file mode 100644
index 0000000..c607883
--- /dev/null
+++ b/backend/src/classes.py
@@ -0,0 +1,9 @@
+from dataclasses import dataclass
+
+
+@dataclass
+class Room:
+    id: int
+    coord: ...
+    name: str
+    pin: int | None