#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#include<bitset>
#include<set>
#include<map>
#include<stack>
#include<queue>
#include<deque>
#include<list>
#include<iomanip>
#include<cmath>
#include<cstring>
#include<functional>
#include<cstdio>
#include<cstdlib>
#include<numeric>
#include<ctime>
//#include<atcoder/all>
using namespace std;
//using namespace atcoder;

#define repr(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define rep(i, n) repr(i, 0, n)
#define INF 2e9
//#define MOD 1000000007
#define MOD 998244353
#define LINF (long long)4e18
#define jck 3.141592
#define PI acos(-1.0)

const double EPS = 1e-10;

using ll = long long;
using Pi = pair<int,int>;
using Pl = pair<ll,ll>;
//using mint = modint998244353;

int dh[] = {1,0,-1,0};
int dw[] = {0,1,0,-1};

ll dp[5010][10010];

int main(){
    int n; cin >> n;
    vector<int> a(n);
    rep(i,n) cin >> a[i];
    int cnt0 = 0,cnt1 = 0;
    vector<Pi> v;
    rep(i,n){
        if(a[i] == 2){
            v.emplace_back(cnt0,cnt1);
            cnt0 = 0;
            cnt1 = 0;
        }
        else if(a[i] == 0) cnt0++;
        else cnt1++;
    }
    v.emplace_back(cnt0,cnt1);

    int m = v.size();
    rep(i,5010)rep(j,10010){
        dp[i][j] = LINF;
    }
    dp[0][5000] = 0;
    rep(i,m){
        int u = v[i].first;
        int w = v[i].second;
        rep(j,10010){
            if(dp[i][j] == LINF) continue;
            dp[i+1][j+u] = min(dp[i+1][j+u],dp[i][j]+u);
            dp[i+1][j-w] = min(dp[i+1][j-w],dp[i][j]);
        }
    }
    if(dp[m][5000] == LINF) cout << -1 << endl;
    else cout << dp[m][5000] << endl;

}