백준 3273

package baekjoon.이분탐색;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.StringTokenizer;

class Main3273 {

   static int N, X, ans;
   static int[] a;

   public static void main(String[] args) throws Exception {
      input();
      go();
      System.out.println(ans);
   }

   private static void go() {
      int L = 0, R = N - 1;
      while (L < R) {
         if (a[L] + a[R] == X) {
            ans++;
            L++;
            R--;
         } else if (a[L] + a[R] > X) {
            R--;
         } else if (a[L] + a[R] < X) {
            L++;
         }
      }
   }

   private static void input() throws IOException {
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
      StringTokenizer st;
      N = Integer.parseInt(br.readLine());
      a = new int[N];
      st = new StringTokenizer(br.readLine());
      for (int i = 0; i < N; i++) {
         a[i] = Integer.parseInt(st.nextToken());
      }
      X = Integer.parseInt(br.readLine());
      Arrays.sort(a);
   }
}