auto update README.md

Author: Ja2Hw

README.md

@@ -156,6 +156,7 @@
 |2798. 블랙잭|[링크](./%EB%B0%B1%EC%A4%80/Bronze/2798.%E2%80%85%EB%B8%94%EB%9E%99%EC%9E%AD/%EB%B8%94%EB%9E%99%EC%9E%AD.py)|
 |28014. 첨탑 밀어서 부수기|[링크](./%EB%B0%B1%EC%A4%80/Bronze/28014.%E2%80%85%EC%B2%A8%ED%83%91%E2%80%85%EB%B0%80%EC%96%B4%EC%84%9C%E2%80%85%EB%B6%80%EC%88%98%EA%B8%B0/README.md)|
 |28074. 모비스|[링크](./%EB%B0%B1%EC%A4%80/Bronze/28074.%E2%80%85%EB%AA%A8%EB%B9%84%EC%8A%A4/README.md)|
+|28115. 등차수열의 합|[링크](./%EB%B0%B1%EC%A4%80/Bronze/28115.%E2%80%85%EB%93%B1%EC%B0%A8%EC%88%98%EC%97%B4%EC%9D%98%E2%80%85%ED%95%A9/README.md)|
 |28235. 코드마스터 2023|[링크](./%EB%B0%B1%EC%A4%80/Bronze/28235.%E2%80%85%EC%BD%94%EB%93%9C%EB%A7%88%EC%8A%A4%ED%84%B0%E2%80%852023/README.md)|
 |28290. 안밖？ 밖안？ 계단？ 역계단？|[링크](./%EB%B0%B1%EC%A4%80/Bronze/28290.%E2%80%85%EC%95%88%EB%B0%96%EF%BC%9F%E2%80%85%EB%B0%96%EC%95%88%EF%BC%9F%E2%80%85%EA%B3%84%EB%8B%A8%EF%BC%9F%E2%80%85%EC%97%AD%EA%B3%84%EB%8B%A8%EF%BC%9F/README.md)|
 |28417. 스케이트보드|[링크](./%EB%B0%B1%EC%A4%80/Bronze/28417.%E2%80%85%EC%8A%A4%EC%BC%80%EC%9D%B4%ED%8A%B8%EB%B3%B4%EB%93%9C/%EC%8A%A4%EC%BC%80%EC%9D%B4%ED%8A%B8%EB%B3%B4%EB%93%9C.py)|