using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = NN;
        var a = NList;
        var list = new List<int>();
        var zeros = 0;
        var ones = 0;
        var zlist = new List<int>();
        for (var i = 0; i < n; ++i)
        {
            if (a[i] == 0) ++zeros;
            else if (a[i] == 1) ++ones;
            if (a[i] == 2 || i + 1 == n)
            {
                if (zeros + ones > 0)
                {
                    list.Add(zeros + ones);
                    zlist.Add(zeros);
                }
                zeros = 0;
                ones = 0;
            }
        }
        var INF = int.MaxValue / 2;
        var dp = Enumerable.Repeat(INF, n + 1).ToArray();
        dp[0] = 0;
        for (var i = 0; i < list.Count; ++i)
        {
            var ndp = Enumerable.Repeat(INF, n + 1).ToArray();
            for (var j = 0; j < dp.Length; ++j)
            {
                ndp[j] = Math.Min(ndp[j], dp[j] + zlist[i]);
                if (j + list[i] <= n) ndp[j + list[i]] = Math.Min(ndp[j + list[i]], dp[j] + list[i] - zlist[i]);
            }
            dp = ndp;
        }
        var sum = zlist.Sum();
        WriteLine(dp[sum] == INF ? -1 : dp[sum] / 2);
    }
}