#include <bits/stdc++.h>
#define rep(i, l, r) for (int i = (l); i < (r); i++)
using namespace std;

typedef long long ll;

int main() {
    int N;
    cin >> N;
    vector<int> A(N, 0), B(1, 0), C(3, 0), D(1, 0);
    rep(i, 0, N) {
        cin >> A[i];
        C[A[i]]++;
        if (A[i] == 2) {
            if (B.back() > 0) {
                B.push_back(0);
                D.push_back(0);
            }
        } else {
            B.back()++;
            D.back() += A[i];
        }
    }
    if (B.back() == 0) {
        B.pop_back();
        D.pop_back();
    }
    int M = B.size();
    //rep(i, 0, M) cout << B[i] << " "; cout << endl;
    //rep(i, 0, M) cout << D[i] << " "; cout << endl;
    vector<vector<int>> dp(M + 1, vector<int>(N + 1, 1e9));
    dp[0][0] = 0;
    rep(i, 0, M) {
        rep(j, 0, N) {
            if (dp[i][j] == 1e9) continue;
            dp[i + 1][j] = min(dp[i + 1][j], dp[i][j] + D[i]);
            dp[i + 1][j + B[i]] = min(dp[i + 1][j + B[i]], dp[i][j] + B[i] - D[i]);
        }
    }
    int ans = dp[M][C[1]];
    if (ans == 1e9) ans = -1;
    else ans /= 2;
    cout << ans << endl;
}