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 meats = new ArrayList<>(); ArrayList 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 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 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(); } }