/*
--------------              |   /
      |                     |  /
      |                     | /
      |             *       |/          |    |         ------            *
      |                     |           |    |        /      \
      |             |       |\          |    |       |       |\          |
   \  |             |       | \         |    |       |       | \         |
    \ |             |       |  \        |    |        \     /   \        |
     V              |       |   \        \__/|         -----     \       |
*/
#include <bits/stdc++.h>
using namespace std;
#define debug(x) cerr << "\e[1;31m" << #x << " = " << (x) << "\e[0m\n"
#define print(x) emilia_mata_tenshi(#x, begin(x), end(x))
template<typename T> void emilia_mata_tenshi(const char *s, T l, T r) {
	cerr << "\e[1;33m" << s << " = [";
	while(l != r) {
		cerr << *l;
		cerr << (++l == r ? ']' : ',');
	}
	cerr << "\e[0m\n";
}

#define EmiliaMyWife ios::sync_with_stdio(0); cin.tie(NULL);
using ll = int64_t;
using ull = uint64_t;
using ld = long double;
using uint = uint32_t;
const double EPS  = 1e-8;
const int INF     = 0x3F3F3F3F;
const ll LINF     = 4611686018427387903;
const int MOD     = 1e9+7;
/*--------------------------------------------------------------------------------------*/

signed main() { EmiliaMyWife
	int n;
	cin >> n;
	vector<int> arr(n);
	for(int i = 0; i < n; i++)
		cin >> arr[i];
	int tot = 0;
	for(int i = 0; i < n; i++)
		if(arr[i] == 1)
			tot++;
	vector<int> can(tot + 1, INF);
	can[0] = 0;
	for(int i = 0, c = 0, b = 0; i <= n; i++) {
		if(i == n || arr[i] == 2) {
			int cost = c - b;
			for(int j = c; j <= tot; j++)
				can[j] = min(can[j], can[j - c] + cost);
			c = 0;
			b = 0;
		}
		else
			c++, b += arr[i] == 1;
	}
	if(can[tot] == INF)
		cout << "-1\n";
	else
		cout << can[tot] << '\n';
}