import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


enum INF = 10^^9;

void main() {
  try {
    for (; ; ) {
      const N = readInt();
      auto A = new int[N];
      foreach (i; 0 .. N) {
        A[i] = readInt();
      }
      
      int len = 0;
      int[] crt = [0];
      for (int i = 0, j; i < N; i = j) {
        for (j = i; j < N && (A[i] != 2) == (A[j] != 2); ++j) {}
        if (A[i] != 2) {
          int[2] cost;
          foreach (s; 0 .. 2) {
            foreach (k; i .. j) {
              if (A[k] != s) {
                ++cost[s];
              }
            }
          }
          auto nxt = new int[len + (j - i) + 1];
          nxt[] = INF;
          foreach (x; 0 .. len + 1) {
            chmin(nxt[x], crt[x] + cost[0]);
            chmin(nxt[x + (j - i)], crt[x] + cost[1]);
          }
          len += (j - i);
          crt = nxt;
          debug {
            writeln("cost = ", cost);
            writeln("crt = ", crt);
          }
        }
      }
      
      int ans = INF;
      const cnt1 = cast(int)(A.count(1));
      if (-len <= cnt1 && cnt1 <= len) {
        ans = crt[cnt1];
      }
      writeln((ans >= INF) ? -1 : (ans / 2));
    }
  } catch (EOFException e) {
  }
}