Pair Summing to K

**Description:** Given n integers (values 0..10^6) and a target k, count how many **unordered pairs** (i < j) satisfy a[i] + a[j] = k. **Input:** Line 1: n k (2 ≤ n ≤ 10^5, 0 ≤ k ≤ 2·10^6). Line 2: n integers. **Output:** One integer — the number of pairs. **Example:** 5 6 / 1 5 2 4 5 → pairs (1,5), (2,4) and (5... wait: values 1+5=6 gives two pairs (with both 5s), 2+4=6 gives one → total 3.