The Missing Number

**Description:** The numbers 1..n should all be present, but exactly one is missing. Print it. **Input:** Line 1: n (2 ≤ n ≤ 10^6). Line 2: n−1 integers — the present ones. **Output:** One integer — the missing number. **Example:** 5 / 1 2 4 5 → 3. *Two solutions exist:* a marking array, or the sum trick (missing = n(n+1)/2 − sum). Both are worth writing.