#include<bits/stdc++.h>
using namespace std;
#define ALL(x) begin(x),end(x)
#define rep(i,n) for(int i=0;i<(n);i++)
#define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define mod 1000000007
using ll=long long;
const int INF=1000000000;
const ll LINF=1001002003004005006ll;
int dx[]={1,0,-1,0},dy[]={0,1,0,-1};
// ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
template<class T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<class T>bool chmin(T &a,const T &b){if(b<a){a=b;return true;}return false;}

struct IOSetup{
    IOSetup(){
        cin.tie(0);
        ios::sync_with_stdio(0);
        cout<<fixed<<setprecision(12);
    }
} iosetup;

template<typename T>
ostream &operator<<(ostream &os,const vector<T>&v){
    for(int i=0;i<(int)v.size();i++) os<<v[i]<<(i+1==(int)v.size()?"":" ");
    return os;
}
template<typename T>
istream &operator>>(istream &is,vector<T>&v){
    for(T &x:v)is>>x;
    return is;
}

signed main(){
    int N;cin>>N;
    vector<int> V(N);
    cin>>V;

    vector<int> W,A,B;
    for(int i=0;i<N;){
        if(V[i]==2){
            i++;
            continue;
        }

        int c=0,a=0,b=0;
        while(i+c<N and V[i+c]!=2){
            if(V[i+c]==0) a++;
            if(V[i+c]==1) b++;
            c++;
        }
        i+=c;
        W.push_back(c);
        A.push_back(a);
        B.push_back(b);
    }

    N=(int)W.size();
    vector<vector<int>> dp(N+1,vector<int>(6000,INF));

    dp[0][0]=0;
    rep(i,N){
        rep(j,6000){
            // put A
            chmin(dp[i+1][j+W[i]],dp[i][j]+B[i]);
            // put B
            chmin(dp[i+1][j],dp[i][j]+A[i]);
        }
    }

    int SA=0;
    for(auto &a:A) SA+=a;

    if(dp[N][SA]<INF) cout<<dp[N][SA]/2<<endl;
    else cout<<-1<<endl;
    return 0;
}