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

public class Main {
    static ArrayList<Integer> meats = new ArrayList<>();
    static ArrayList<Integer> grasses = new ArrayList<>();
    static ArrayList<HashMap<Integer, HashMap<Integer, Integer>>> dp = new ArrayList<>();
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int n = sc.nextInt();
        int mCount = 0;
        int gCount = 0;
        for (int i = 0; i < n; i++) {
            int x = sc.nextInt();
            if (x == 0) {
                mCount++;
            } else if (x == 1) {
                gCount++;
            } else {
                if (mCount > 0 || gCount > 0) {
                    meats.add(mCount);
                    grasses.add(gCount);
                    dp.add(new HashMap<>());
                    mCount = 0;
                    gCount = 0;
                }
            }
        }
        if (mCount > 0 || gCount > 0) {
            meats.add(mCount);
            grasses.add(gCount);
            dp.add(new HashMap<>());
        }
        int ans = dfw(dp.size() - 1, 0, 0);
        if (ans > n * 2) {
            System.out.println(-1);
        } else {
            System.out.println(ans / 2);
        }
    }
    
    static int dfw(int idx, int m, int g) {
        if (idx < 0) {
            if (m == 0 && g == 0) {
                return 0;
            } else {
                return Integer.MAX_VALUE / 2;
            }
        }
        if (!dp.get(idx).containsKey(m)) {
            dp.get(idx).put(m, new HashMap<>());
        }
        if (!dp.get(idx).get(m).containsKey(g)) {
            dp.get(idx).get(m).put(g, Math.min(dfw(idx - 1, m - grasses.get(idx), g + grasses.get(idx)) + grasses.get(idx), dfw(idx - 1, m + meats.get(idx), g - meats.get(idx)) + meats.get(idx)));
        }
        return dp.get(idx).get(m).get(g);
    }
}
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();
    }
    
}