#include<algorithm>
#include<iostream>
#include<vector>
#include<map>
#include<cassert>
#include<set>
using namespace std;
using ll = long long;

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    int n;
    cin>>n;
    vector<int> a(n);
    for(int i = 0;i<n;i++) cin>>a[i];
    vector<vector<int>> dp(n+1,vector<int>(3,1e9));
    dp[0][2] = 0;
    for(int i = 0;i<n;i++){
        vector<vector<int>> ndp(n+1,vector<int>(3,1e9));
        if(a[i]==2){
            for(int j = 0;j<=n;j++){
                ndp[j][2] = min(ndp[j][2],dp[j][0]);
                ndp[j][2] = min(ndp[j][2],dp[j][1]);
                ndp[j][2] = min(ndp[j][2],dp[j][2]);
            }
            swap(ndp,dp);
            continue;
        }
        for(int j = 0;j<=n;j++){
            if(a[i]==0){
                if(j+1<=n) ndp[j+1][0] = min(ndp[j+1][0],dp[j][0]);
                if(j+1<=n) ndp[j+1][0] = min(ndp[j+1][0],dp[j][2]);
                ndp[j][1] = min(ndp[j][1],dp[j][1]+1);
                ndp[j][1] = min(ndp[j][1],dp[j][2]+1);
            }else{
                if(j+1<=n) ndp[j+1][0] = min(ndp[j+1][0],dp[j][0]+1);
                if(j+1<=n) ndp[j+1][0] = min(ndp[j+1][0],dp[j][2]+1);
                ndp[j][1] = min(ndp[j][1],dp[j][1]);
                ndp[j][1] = min(ndp[j][1],dp[j][2]);
            }
        }
        swap(ndp,dp);
    }
    int ans = 1e9;
    int cnt = 0;
    for(int i = 0;i<n;i++) if(a[i]==0) cnt++;
    ans = dp[cnt][0];
    ans = min(ans,dp[cnt][1]);
    ans = min(ans,dp[cnt][2]);
    if(ans==1e9) ans = -1;
    else ans = ans / 2;
    cout<<ans<<endl;
}