diff --git a/dohyeondol1/README.md b/dohyeondol1/README.md
index f44924f..26f1ff5 100644
--- a/dohyeondol1/README.md
+++ b/dohyeondol1/README.md
@@ -21,3 +21,4 @@
  | 17차시 | 2025.06.26 |  기하학  | [선분 교차 2](https://www.acmicpc.net/problem/17387)|https://github.com/AlgoLeadMe/AlgoLeadMe-13/pull/65|
  | 18차시 | 2025.07.04 |  기하학  | [Water Testing](https://www.acmicpc.net/problem/16057)|https://github.com/AlgoLeadMe/AlgoLeadMe-13/pull/70|
  | 19차시 | 2025.07.09 |  그래프 이론  | [최소 스패닝 트리](https://www.acmicpc.net/problem/1197)|https://github.com/AlgoLeadMe/AlgoLeadMe-13/pull/76|
+ | 20차시 | 2025.07.10 |  문자열  | [접두사](https://www.acmicpc.net/problem/1141)|https://github.com/AlgoLeadMe/AlgoLeadMe-13/pull/78|
diff --git "a/dohyeondol1/\353\254\270\354\236\220\354\227\264/20-dohyeondol1.cpp" "b/dohyeondol1/\353\254\270\354\236\220\354\227\264/20-dohyeondol1.cpp"
new file mode 100644
index 0000000..3f713dd
--- /dev/null
+++ "b/dohyeondol1/\353\254\270\354\236\220\354\227\264/20-dohyeondol1.cpp"
@@ -0,0 +1,26 @@
+#include <iostream>
+#include <string>
+#include <algorithm>
+using namespace std;
+
+string arr[50];
+
+int main() {
+    cin.tie(nullptr)->sync_with_stdio(false);
+    
+    int N;
+    cin >> N;
+    for(int i = 0; i < N; i++)
+        cin >> arr[i];
+    sort(arr, arr+N);
+    
+    int count = N;
+    for(int i = 0; i < N-1; i++) {
+        string prefix = arr[i+1].substr(0, arr[i].size());
+        if(prefix == arr[i])
+            count--;
+    }
+
+    cout << count << '\n';
+    return 0;
+}
\ No newline at end of file