diff --git a/Lee-JoungHyun/BOJ_15961.java b/Lee-JoungHyun/BOJ_15961.java
new file mode 100644
index 0000000..85bb9dc
--- /dev/null
+++ b/Lee-JoungHyun/BOJ_15961.java
@@ -0,0 +1,57 @@
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.util.StringTokenizer;
+
+public class BOJ_15961 {
+    static int N, c, k, d, arr[];
+    static int[] visit = new int[3001];
+
+    public static void main(String[] args) throws IOException {
+        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+        StringTokenizer st = null;
+        st = new StringTokenizer(br.readLine());
+        N = Integer.parseInt(st.nextToken());
+        d = Integer.parseInt(st.nextToken());
+        k = Integer.parseInt(st.nextToken());
+        c = Integer.parseInt(st.nextToken());
+        arr = new int[N];
+        for (int i = 0; i < N; i++) {
+            arr[i] = Integer.parseInt(br.readLine());
+        }
+
+        int cnt = 0;
+        int start = 0;
+        int end = k;
+        int answer = 1;
+
+        for (int i = 0; i < end; i++) {
+            if (visit[arr[i]] == 0) {
+                cnt++;
+            }
+            visit[arr[i]]++;
+        }
+        visit[c] += 1;
+        // 전에 C가 있었음 = cnt에 반영이 되어있음
+        answer = visit[c] > 1 ? cnt : ++cnt;
+        //System.out.println("0, 4 -> " + answer);
+        while (start <= N-1) {
+
+            if (--visit[arr[start]] == 0) {
+                cnt--;
+            }
+            end++;
+            if (visit[arr[(end - 1) % N]] == 0) {
+                cnt++;
+            }
+            visit[arr[(end - 1) % N]]++;
+            start++;
+
+            answer = Math.max(answer, cnt);
+            //System.out.println(start + ", " + end + " -> " + answer + "-" + cnt);
+            if (answer == k+1)
+                break;
+        }
+        System.out.println(answer);
+    }
+}
diff --git a/Lee-JoungHyun/BOJ_18866.java b/Lee-JoungHyun/BOJ_18866.java
new file mode 100644
index 0000000..6859ab5
--- /dev/null
+++ b/Lee-JoungHyun/BOJ_18866.java
@@ -0,0 +1,61 @@
+import java.io.BufferedReader;
+import java.io.InputStreamReader;
+import java.util.*;
+ 
+public class BOJ_18866. {
+
+    public static void main(String[] args) throws Exception{
+ 
+        final int MAX = 1_000_000_001;
+        final int MIN = -1;
+ 
+        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+        StringTokenizer st;
+ 
+        int N = Integer.parseInt(br.readLine());
+ 
+        int[][] arr = new int[N + 1][2];
+ 
+        int[] minHappyLeft = new int[N + 2]; // 구간 1 ~ index의 최소 행복도
+        int[] maxHappyRight = new int[N + 2]; // 구간 index ~ N의 최대 행복도
+ 
+        int[] maxFatigueLeft = new int[N + 2]; // 구간 1 ~ index의 최대 피로도
+        int[] minFatigueRight = new int[N + 2]; // 구간 index ~ N의 최소 피로도
+ 
+ 
+        minHappyLeft[0] = MAX;
+        maxHappyRight[N + 1] = MIN;
+        maxFatigueLeft[0] = MIN;
+        minFatigueRight[N + 1] = MAX;
+ 
+        for (int i = 1; i <= N; i++) {
+            st = new StringTokenizer(br.readLine());
+            arr[i][0] = Integer.parseInt(st.nextToken());
+            arr[i][1] = Integer.parseInt(st.nextToken());
+ 
+            if(arr[i][0] != 0) minHappyLeft[i] = Math.min(minHappyLeft[i - 1], arr[i][0]);
+            else minHappyLeft[i] = minHappyLeft[i - 1];
+            if(arr[i][1] != 0) maxFatigueLeft[i] = Math.max(maxFatigueLeft[i - 1], arr[i][1]);
+            else maxFatigueLeft[i] = maxFatigueLeft[i - 1];
+        }
+      
+        for (int i = N; i >= 1; i--) {
+            if(arr[i][0] != 0)  maxHappyRight[i] = Math.max(maxHappyRight[i + 1], arr[i][0]);
+            else maxHappyRight[i] = maxHappyRight[i + 1];
+            if(arr[i][1] != 0) minFatigueRight[i] = Math.min(minFatigueRight[i + 1], arr[i][1]);
+            else minFatigueRight[i] = minFatigueRight[i + 1];
+        }
+
+        for (int i = N - 1; i >= 1; i--) {
+            // 1 ~ i년까지의 최소 행복도가 i+1 ~ N년의 최대 행복도보다 커야하며,
+            // 1 ~ i년까지의 최대 피로도가 i+1 ~ n년의 최소 피로도보다 작아야 한다.
+            if(minHappyLeft[i] >= maxHappyRight[i + 1] && maxFatigueLeft[i] <= minFatigueRight[i + 1]) {
+                System.out.println(i);
+                return;
+            }
+        }
+ 
+        // 찾지 못한 경우
+        System.out.println(-1);
+    }
+}