import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int n = sc.nextInt();
        ArrayList<Integer> meat = new ArrayList<>();
        ArrayList<Integer> bege = new ArrayList<>();
        int total = 0;
        int count = 0;
        for (int i = 0; i < n; i++) {
            int x = sc.nextInt();
            if (x == 2 && total > 0) {
                meat.add(count);
                bege.add(total - count);
            }
            if (x == 2) {
                count = 0;
                total = 0;
                continue;
            }
            total++;
            count += x;
        }
        if (total > 0 && count > 0 && count < total) {
            meat.add(count);
            bege.add(total - count);
        }
        int length = meat.size();
        ArrayList<HashMap<Integer, Integer>> dp = new ArrayList<>();
        for (int i = 0; i <= length; i++) {
            dp.add(new HashMap<>());
        }
        dp.get(0).put(0, 0);
        for (int i = 1; i <= length; i++) {
            int m = meat.get(i - 1);
            int b = bege.get(i - 1);
            for (int x : dp.get(i - 1).keySet()) {
                dp.get(i).put(x + m, Math.min(dp.get(i).getOrDefault(x + m, Integer.MAX_VALUE), dp.get(i - 1).get(x) + m));
                dp.get(i).put(x - b, Math.min(dp.get(i).getOrDefault(x - b, Integer.MAX_VALUE), dp.get(i - 1).get(x)));
            }
        }
        System.out.println(dp.get(length).getOrDefault(0, -1));
    }
}

class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public double nextDouble() throws Exception {
        return Double.parseDouble(next());
    }
    
    public String next() throws Exception {
        if (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
}