import java.io.*;
import java.util.*;
/**
 * CCC '12 S4 - A Coin Game
 * Question type: Graph Theory
 * 50/50 on DMOJ
 * Question URL: https://dmoj.ca/problem/ccc12s4
 * @author Tommy Pang
 */
public class j5
{
    static Scanner input = new Scanner(System.in);
    static int n;
    static Map<String, String> vis;

    public static void main(String[] args) throws IOException
    {
        n = readInt();
        do {
            vis = new HashMap<>();
            List<Integer> [] table = new List[n];
            for (int i = 0; i < n; i++)
            {
                table[i] = new ArrayList<>();
                table[i].add(readInt());
            }
            int ans = BFS(table);
            System.out.println(ans==-1 ? "IMPOSSIBLE" : ans);
            n = readInt();
        }
        while (n!=0);
    }

    public static int BFS(List<Integer> [] list)
    {
        Queue<List<Integer> []> queue = new LinkedList<>();
        Queue<Integer> dis = new LinkedList<>();

        queue.add(list);        // Queue with states
        dis.add(0);             // Parallel queue: steps (distance of each state)

        while (!queue.isEmpty())
        {
            // Get next state from queue
            List<Integer> [] cur = queue.poll();
            int d = dis.poll();

	    // DEBUG
	    print(cur, d);

            // Is cur a finished state ?
            if (finished(cur)) return d;

            for (int i = 0; i < n-1; i++)
            {
                if (cur[i].size()==0) continue;
                List<Integer> [] nxt = clone(cur);
                if (nxt[i].get(0)<(nxt[i+1].size()==0?Integer.MAX_VALUE:nxt[i+1].get(0)))
                {
                    nxt[i+1].add(0, nxt[i].remove(0));
                    if (!visited(nxt))
                    {
                        queue.add(nxt);
                        dis.add(d+1);
                    }
                }
            }

            for (int i = n-1; i > 0; i--)
            {
                if (cur[i].size()==0) continue;

                List<Integer> [] nxt = clone(cur);

                if (nxt[i].get(0)<(nxt[i-1].size()==0?Integer.MAX_VALUE:nxt[i-1].get(0)))
//                                 ^^^^^^^^^^^^^^^^^^ position is OPEN
                {
                    nxt[i-1].add(0, nxt[i].remove(0)); // Move top coin i->i-1
                    if (!visited(nxt))
                    {
                        queue.add(nxt);
                        dis.add(d+1);           // Increase distance
                    }
                }
            }
        }
        return -1;
    }


    public static boolean visited(List<Integer> [] table)
    {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < n; i++)
        {
            List<Integer> col = table[i];
            for (int nxt : col)
            {
                sb.append(nxt);
            }
            if (i!=n-1) sb.append("-");
        }
        if (!vis.containsKey(sb.toString()))
        {
            vis.put(sb.toString(), sb.toString());
            return false;
        }
        else return true;
    }

    public static boolean finished(List<Integer> [] table)
    {
        for (int i = 0; i < n; i++)
        {
            if (table[i].size()!=1)
                return false;   // More than 1 coin in a spot
            else if (table[i].get(0)!=i+1)
                return false;   // Coin not in its correct position
        }
        return true;
    }

    // Clone a state (table entry)
    public static List<Integer> [] clone(List<Integer> [] table)
    {
        List<Integer> [] ret = new List[table.length];
        for (int i = 0; i < table.length; i++)
        {
            ret[i] = new ArrayList<>(table[i]);
        }
        return ret;
    }

    public static void print(List<Integer> [] table, int d)
    {
        List<Integer> x;
        for (int i = 0; i < table.length; i++)
        {
            x = table[i];
            System.out.print(x + " ");
        }
        System.out.println(": " + d);
    }



    static int readInt() throws IOException
    {
        return input.nextInt();
    }
}