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();
        int zeroCount = 0;
        int oneCount = 0;
        ArrayList<Integer> zeros = new ArrayList<>();
        ArrayList<Integer> ones = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            int x = sc.nextInt();
            if (x == 2) {
                if (zeroCount > 0 || oneCount > 0) {
                    zeros.add(zeroCount);
                    ones.add(oneCount);
                    zeroCount = 0;
                    oneCount = 0;
                }
            } else if (x == 0) {
                zeroCount++;
            } else {
                oneCount++;
            }
        }
        if (zeroCount > 0 || oneCount > 0) {
            zeros.add(zeroCount);
            ones.add(oneCount);
        }
        int total = 0;
        int size = zeros.size();
        for (int x : zeros) {
            total += x;
        }
        int[] dp = new int[total + 1];
        Arrays.fill(dp, Integer.MAX_VALUE);
        dp[0] = 0;
        for (int i = 0; i < size; i++) {
            int sum = zeros.get(i) + ones.get(i);
            int value = ones.get(i);
            for (int j = total; j - sum >= 0; j--) {
                if (dp[j - sum] == Integer.MAX_VALUE) {
                    continue;
                }
                dp[j] = Math.min(dp[j], dp[j - sum] + value);
            }
        }
        if (dp[total] == Integer.MAX_VALUE) {
            System.out.println(-1);
        } else {
            System.out.println(dp[total]);
        }
    }
}
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 String next() throws Exception {
        if (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
}