/* FILE: collage_selfcertifying_feasibility.cpp   last change: 20-Sept-2012   author: Romeo Rizzi
 * a solver for problem "collage" in the 28-Sept-2012 exam in Algorithms.
 */

//#define NDEBUG // NDEBUG disabilita tutte le assert se definita prima di includere <cassert> 
#include <cassert>

#ifndef NDEBUG
#  include <iostream>  // non voglio includerla quando Non compilo in modalita' DEBUG 
#endif

#include <fstream>

using namespace std;

const int MAX_N = 1000; int n;
int color[MAX_N];

int memo[MAX_N +1][MAX_N +1];
const int UNKNOWN = -1;
const int LOOK = -2;
const int DROP = -1;

void ric(int i, int j, bool opened = false) {
        if (i>j) return;
        if (!opened)
                cout << color[i] << " " << LOOK << " ";
        if( memo[i][j] == 1 + memo[i+1][j] )
     { cout << DROP << " "; ric(i+1,j); return; }
    int sec;
    for (sec = 1; i+sec <= j; ++sec)
        if ((color[i+sec] == color[i]) &&
                (memo[i][j] == memo[i+1][i+sec-1] + memo[i+sec][j])) {
                            
                    ric(i+1, i+sec-1);
                cout << LOOK << " ";
                    ric(i+sec, j, true);
                        return;
        }
}

int main (int argc, char** argv) {
	ifstream in("input.txt"); assert( in );
	in >> n;
      	for(int i = 0; i < n; ++i)  in >> color[i];
	in.close();
	
	for(int i = n-1; i >= 0; --i)
	   for(int j = 0; j < n; ++j)
	       if( i>j ) memo[i][j] = 0;
	       else {
		   memo[i][j] = 1 + memo[i+1][j];
		   for(int sec = 1; i + sec <= j; ++sec)
		       if( color[i+sec] == color[i] )
			 memo[i][j] = min(memo[i][j], memo[i+1][i+sec-1] + memo[i+sec][j]);
	       }
	
	ofstream out("output.txt"); assert( out );
	out << memo[0][n-1] << endl;
	out.close();

	ric(0, n-1);
	
	return 0;
}