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

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int n = sc.nextInt();
        ArrayList<Integer> meats = new ArrayList<>();
        ArrayList<Integer> grasses = new ArrayList<>();
        int a = 0;
        int b = 0;
        for (int i = 0; i < n; i++) {
            int x = sc.nextInt();
            if (x == 0) {
                a++;
            } else if (x == 1) {
                b++;
            } else {
                if (a == 0 && b == 0) {
                    continue;
                }
                meats.add(a);
                grasses.add(b);
                a = 0;
                b = 0;
            }
        }
        if (a > 0 || b > 0) {
            meats.add(a);
            grasses.add(b);
        }
        HashMap<Integer, Integer> current = new HashMap<>();
        current.put(0, 0);
        for (int i = 0; i < meats.size(); i++) {
            int x = meats.get(i);
            int y = grasses.get(i);
            HashMap<Integer, Integer> next = new HashMap<>();
            for (int z : current.keySet()) {
                next.put(z - x, Math.min(next.getOrDefault(z - x, Integer.MAX_VALUE), current.get(z)));
                next.put(z + y, Math.min(next.getOrDefault(z + y, Integer.MAX_VALUE), current.get(z) + y));
            }
            current = next;
        }
        System.out.println(current.getOrDefault(0, -1));
    }
}
class Utilities {
    static String arrayToLineString(Object[] arr) {
        return Arrays.stream(arr).map(x -> x.toString()).collect(Collectors.joining("\n"));
    }
    
    static String arrayToLineString(int[] arr) {
        return String.join("\n", Arrays.stream(arr).mapToObj(String::valueOf).toArray(String[]::new));
    }
}
class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    StringBuilder sb = new StringBuilder();
    
    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 int[] nextIntArray() throws Exception {
        return Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();
    }
    
    public String next() throws Exception {
        while (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
    
}