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+```java
+import java.io.*;
+import java.util.*;
+
+public class boj1111 {
+    static BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+    static StringTokenizer st;
+    static void nextLine() throws Exception {st = new StringTokenizer(br.readLine());}
+    static int nextInt() {return Integer.parseInt(st.nextToken());}
+
+    public static void main(String[] args) throws Exception {
+        nextLine();
+        int N = nextInt();
+        nextLine();
+        int[] num = new int[N];
+        for (int i = 0; i < N; i++) num[i] = nextInt();
+        if (N == 1) {
+            System.out.println("A");
+            return;
+        } else if (N == 2) {
+            if (num[0] == num[1]) System.out.println(num[0]);
+            else System.out.println("A");
+            return;
+        }
+        int a = (num[1] != num[0]) ? (num[2] - num[1]) / (num[1] - num[0]) : 0;
+        int b = num[1] - num[0] * a;
+
+        for (int i = 1; i < N-1; i++) {
+            if (num[i] * a + b != num[i+1]) {
+                System.out.println("B");
+                return;
+            }
+        }
+        System.out.println(num[N-1] * a + b);
+    }
+}
+```