diff --git a/resource/runner/problem/book/ch2/4. knumber/data/output/3.output b/resource/runner/problem/book/ch2/4. knumber/data/output/3.output
index b93fc8c..3a2e3f4 100644
--- a/resource/runner/problem/book/ch2/4. knumber/data/output/3.output	
+++ b/resource/runner/problem/book/ch2/4. knumber/data/output/3.output	
@@ -1 +1 @@
-NO RESULT
+-1
diff --git a/resource/runner/problem/book/ch2/4. knumber/data/output/5.output b/resource/runner/problem/book/ch2/4. knumber/data/output/5.output
index b93fc8c..3a2e3f4 100644
--- a/resource/runner/problem/book/ch2/4. knumber/data/output/5.output	
+++ b/resource/runner/problem/book/ch2/4. knumber/data/output/5.output	
@@ -1 +1 @@
-NO RESULT
+-1
diff --git a/resource/runner/problem/book/ch2/4. knumber/data/output/8.output b/resource/runner/problem/book/ch2/4. knumber/data/output/8.output
index b93fc8c..3a2e3f4 100644
--- a/resource/runner/problem/book/ch2/4. knumber/data/output/8.output	
+++ b/resource/runner/problem/book/ch2/4. knumber/data/output/8.output	
@@ -1 +1 @@
-NO RESULT
+-1
diff --git a/resource/runner/problem/book/ch2/4. knumber/description.md b/resource/runner/problem/book/ch2/4. knumber/description.md
index b96007a..31022a7 100644
--- a/resource/runner/problem/book/ch2/4. knumber/description.md	
+++ b/resource/runner/problem/book/ch2/4. knumber/description.md	
@@ -7,12 +7,11 @@
 
 ## Description
 
-给出 $N$ 个数，找出第 $P$ 小的数
+给出 $N$ 个正数，找出第 $P$ 小的数
 
 输入包括两行，第一行为 $N$ 和 $P$，第二行为 $N$ 个数
 
-输出第 $P$ 小的数
-
+输出第 $P$ 小的数，若不存在则输出 $-1$
 
 ## Example Cases
 
@@ -30,3 +29,19 @@
 ```
 2
 ```
+
+### Case 2
+
+#### Input
+
+```
+3 2
+1 1 1
+```
+
+#### Output
+
+```
+-1
+```
+