import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] arr = new int[n];
        int[] lefts = new int[n];
        for (int i = 0; i < n; i++) {
            arr[i] = sc.nextInt();
            if (i == 0) {
                lefts[i] = arr[i];
            } else {
                lefts[i] = Math.min(arr[i], lefts[i - 1]);
            }
        }
        int[] rights = new int[n];
        rights[n - 1] = arr[n - 1];
        for (int i = n - 2; i >= 0; i--) {
            rights[i] = Math.min(arr[i], rights[i + 1]);
        }
        int min = Integer.MAX_VALUE;
        for (int i = 1; i < n - 1; i++) {
            int front = lefts[i - 1];
            int rear = rights[i + 1];
            if ((arr[i] > front && arr[i] > rear) || (arr[i] < front && arr[i] < rear)) {
                min = Math.min(min, arr[i] + rear + front);
            }
        }
        if (min == Integer.MAX_VALUE) {
            System.out.println(-1);
        } else {
            System.out.println(min);
        }
    }
}