#include <bits/stdc++.h>
using namespace std;

int main() {
    int N;
    cin >> N;
    vector<int> A(N);
    for (int i = 0; i < N; i++) cin >> A[i];

    vector<int> len, cnt;
    int s = -1;
    int c = 0;
    for (int i = 0; i <= N; i++) {
        if (i == N || A[i] == 2) {
            len.push_back(i - s - 1);
            cnt.push_back(c);
            s = i;
            c = 0;
        }
        if (i != N && A[i] == 1) c++;
    }

    int M = len.size();
    vector<vector<int>> dp(M+1, vector<int>(N+1, 1 << 30));
    dp[0][0] = 0;
    for (int i = 0; i < M; i++) {
        for (int j = N; j >= 0; j--) {
            if (j - len[i] >= 0 && dp[i][j-len[i]] != 1 << 30) {
                dp[i+1][j] = min(dp[i+1][j], dp[i][j-len[i]] + cnt[i]);
            }
            dp[i+1][j] = min(dp[i+1][j], dp[i][j]);
        }
    }

    int count = 0;
    for (int i = 0; i < N; i++) if (A[i] == 0) count++;

    int ans = dp[M][count];
    if (ans == 1 << 30) ans = -1;
    
    cout << ans << endl;
}