/* -*- coding: utf-8 -*-
 *
 * 1606.cc:  No.1606 Stuffed Animals Keeper - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 5000;
const int INF = 1 << 30;

/* typedef */

/* global variables */

int as[MAX_N], cs[2], ls[MAX_N], lcs[MAX_N][2];
int dp[MAX_N + 1][MAX_N + 1];

/* subroutines */

inline void setmin(int &a, int b) { if (a > b) a = b; }

/* main */

int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) scanf("%d", as + i);

  int m = 0;
  for (int i = 0; i < n;) {
    while (i < n && as[i] == 2) i++;
    if (i >= n) break;

    int j = i;
    while (j < n && as[j] != 2) {
      cs[as[j]]++;
      lcs[m][as[j]]++;
      j++;
    }

    ls[m] = j - i;
    m++;
    i = j;
  }
  //for (int i = 0; i < m; i++)
  //printf("%d(%d,%d) ", ls[i], lcs[i][0], lcs[i][1]); putchar('\n');

  for (int i = 0; i <= m; i++) fill(dp[i], dp[i] + cs[0] + 1, INF);
  dp[0][0] = 0;

  for (int i = 0; i < m; i++)
    for (int j = 0; j <= cs[0]; j++)
      if (dp[i][j] < INF) {
	// -> 0
	if (j + ls[i] <= cs[0])
	  setmin(dp[i + 1][j + ls[i]], dp[i][j] + lcs[i][1]);

	// -> 1
	setmin(dp[i + 1][j], dp[i][j] + lcs[i][0]);
      }

  printf("%d\n", (dp[m][cs[0]] < INF) ? dp[m][cs[0]] / 2 : -1);

  return 0;
}