#include<bits/stdc++.h>
using namespace std;
#define int long long
#define REP(i,m,n) for(int i=(m);i<(n);i++)
#define rep(i,n) REP(i,0,n)
#define pb push_back
#define all(a) a.begin(),a.end()
#define rall(c) (c).rbegin(),(c).rend()
#define mp make_pair
#define endl '\n'
//#define vec vector<ll>
//#define mat vector<vector<ll> >
#define fi first
#define se second
#define double long double
typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll,ll> pll;
//typedef long double ld;
typedef complex<double> Complex;
const ll INF=1e9+7;
const ll MOD=998244353;
const ll inf=INF*INF;
const ll mod=INF;
const ll MAX=1000010;
const double PI=acos(-1.0);
typedef vector<vector<ll> > mat;
typedef vector<ll> vec;





signed main(){
    ll n;cin>>n;
    vector<ll>a(n);
    rep(i,n)cin>>a[i];
    ll cnt=0;
    ll cn=0;
    vector<ll>b(0);
    vector<ll>c(0);
    ll cnt2=0;
    ll cnt0=0;
    ll cnt1=0;
    rep(i,n){
        if(a[i]==2){
            b.pb(cnt);
            c.pb(cn);
            cn=0;
            cnt=0;
            cnt2++;
        }else{
            if(a[i]==0)cn++;
            cnt++;
            if(a[i]==1)cnt1++;
            if(a[i]==0)cnt0++;
        }
    }
    if(cnt1==0||cnt0==0){
        cout<<0<<endl;
        return 0;
    }
    if(cnt2==0){
        cout<<-1<<endl;
        return 0;
    }
    b.pb(cnt);
    c.pb(cn);
    vector<vector<ll> >dp(b.size()+1,vector<ll>(2*n,inf));
    dp[0][n]=0;
    rep(i,b.size()){
        rep(j,2*n){
            if(dp[i][j]==inf)continue;
            ll x=c[i];
            ll y=b[i]-c[i];
            dp[i+1][j+x]=min(dp[i+1][j+x],dp[i][j]+x);
            dp[i+1][j-y]=min(dp[i+1][j-y],dp[i][j]+y);
        }
    }
    if(dp[b.size()][n]==inf){
        cout<<-1<<endl;
    }else{
        cout<<dp[b.size()][n]/2<<endl;
    }
}