import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Scanner;

public class Secv {
//in: 8
// 2 1 3 2 1 3 4 5
//out: 7
//	Singura subsecventa care respecta conditile din enunt este 1 3 2 1 3 4 5
//	Acesta subsecventa contine subsirul 1 2 3 4 5.
static final int NMAX = 5015;
	static ArrayList<Integer>[] pos = new ArrayList[NMAX];
	static int n;
	static int[] a = new int[NMAX];
	static int[] dp = new int[NMAX];

	static HashMap<Integer, Integer> mp = new HashMap<>();

	public static void main(String[] args) throws FileNotFoundException {
		Scanner fin = new Scanner(new File("secv.in"));
		PrintWriter fout = new PrintWriter(new File("secv.out"));
		//Scanner fin = new Scanner(System.in);
		n = fin.nextInt();
		int[] st = new int[n + 1];
		for (int i = 1; i <= n; i++) {
			st[i] = fin.nextInt();
			a[i] = st[i];
			mp.put(a[i], i);
		}

		int cnt = 0;
		for (Integer entry : mp.keySet()) {
			mp.put(entry, ++cnt);
		}

		for (int i = 1; i <= n; i++) {
			a[i] = mp.get(a[i]);
		}

		for (int i = 1; i <= n; i++) {
			if (pos[a[i]] == null) {
				pos[a[i]] = new ArrayList<>();
			}
		}

		for (int i = 1; i <= n; i++) {
			if (a[i] == 1) {
				pos[1].add(i);
				dp[i] = 1;
			} else {
				if (pos[a[i] - 1].size() > 0) {
					int loc = pos[a[i] - 1].get(pos[a[i] - 1].size() - 1);
					dp[i] = dp[loc] + (i - loc);
					pos[a[i]].add(i);
				}
			}
		}

		long ans = Long.MAX_VALUE;
		for (int i = 1; i <= n; i++) {
			if (a[i] == cnt && dp[i] > 0) {
				ans = Math.min(ans, dp[i]);
			}
		}

		fout.println(ans == Long.MAX_VALUE ? -1 : ans);
		//System.out.println(ans == Long.MAX_VALUE ? -1 : ans);
		fin.close();
	}
}