#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=(n)-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=1e9+10;
const ll INF=4e18;
const ll dy[8]={-1,0,1,0,1,1,-1,-1};
const ll dx[8]={0,-1,0,1,1,-1,1,-1};
template <typename T> inline bool chmax(T &a, T b) {
  return ((a < b) ? (a = b, true) : (false));
}
template <typename T> inline bool chmin(T &a, T b) {
  return ((a > b) ? (a = b, true) : (false));
}

int main(){
  ll n;cin >> n;
  vl v(n);rep(i,n)cin >> v[i];v.push_back(2);
  vpl item;
  ll a=0,b=0;
  rep(i,n+1){
    if(v[i]==0)a++;
    else if(v[i]==1)b++;
    else{
      if(a==0&&b==0)continue;
      item.push_back({a,b});
      a=0;b=0;
    }
  }
  ll m=item.size();
  vvl dp(m+1,vl(5010,inf));
  dp[0][0]=0;
  ll zero=0;
  rep(i,m){
    zero+=item[i].first;
    rep(j,5010){
      if(dp[i][j]==inf)continue;
      ll al=item[i].fi+item[i].se;
      chmin(dp[i+1][j+al],dp[i][j]+item[i].se);
      chmin(dp[i+1][j],dp[i][j]+item[i].fi);
    }
  }
  if(dp[m][zero]==inf)cout << -1 <<endl;
  else cout << dp[m][zero]/2 <<endl;
}