Even Number of Tolls

**Bài toán.** Undirected weighted graph; find the minimum-cost walk from 1 to n that uses an **even number of edges** (revisiting allowed). **Constraints:** 1 ≤ n ≤ 100000; 1 ≤ m ≤ 200000; 1 ≤ w ≤ 10^9. **Output:** the minimum cost, or −1 if impossible.