diff --git a/hadongun/README.md b/hadongun/README.md
index 02d568e..1594523 100644
--- a/hadongun/README.md
+++ b/hadongun/README.md
@@ -19,5 +19,6 @@
  | 15차시| 2025.05.21 |  트리  | [트리] (https://www.acmicpc.net/problem/1068)|https://github.com/AlgoLeadMe/AlgoLeadMe-13/pull/58|
  | 16차시| 2025.06.27 |  구현  | [킹] (https://www.acmicpc.net/problem/1063)|https://github.com/AlgoLeadMe/AlgoLeadMe-13/pull/68|
  | 17차시| 2025.07.04 |  두 포인터  | [두 수의 합] (https://www.acmicpc.net/problem/3273)|https://github.com/AlgoLeadMe/AlgoLeadMe-13/pull/71|
- | 18차시| 2025.07.08 |  그래프 탐색  | [헌내기는 친구가 필요해] (https://www.acmicpc.net/problem/21736)||
+ | 18차시| 2025.07.08 |  그래프 탐색  | [헌내기는 친구가 필요해] (https://www.acmicpc.net/problem/21736)|https://github.com/AlgoLeadMe/AlgoLeadMe-13/pull/75|
+ | 19차시| 2025.07.11 |  문자열  | [IOIOI] (https://www.acmicpc.net/problem/5525)||
  ---
\ No newline at end of file
diff --git "a/hadongun/\353\254\270\354\236\220\354\227\264/IOIOI.py" "b/hadongun/\353\254\270\354\236\220\354\227\264/IOIOI.py"
new file mode 100644
index 0000000..7c47068
--- /dev/null
+++ "b/hadongun/\353\254\270\354\236\220\354\227\264/IOIOI.py"
@@ -0,0 +1,31 @@
+import sys
+input = sys.stdin.readline
+
+N = int(input())
+s_len = int(input())
+s = list(input().strip())
+
+i = 0
+count = 0
+
+while i < s_len - (2 * N):
+    if s[i] == 'I':
+        is_pass = True
+        for j in range(N):
+            
+            if s[i + 2*j + 1] != 'O':
+                is_pass = False
+                break
+            
+            if s[i + 2*j + 2] != 'I':
+                is_pass = False
+                break
+        if is_pass:
+            count += 1
+            i += 2  
+        else:
+            i += 1
+    else:
+        i += 1
+
+print(count)