#include <bits/stdc++.h>
#define rep(i, l, r) for(int i = (l); i <= (r); i++)
#define per(i, r, l) for(int i = (r); i >= (l); i--)
#define mem(a, b) memset(a, b, sizeof a)
#define For(i, l, r) for(int i = (l), i##e = (r); i < i##e; i++)
#define pb push_back
#define eb emplace_back
#define all(x) (x).begin(), (x).end()
#define SZ(x) int((x).size())

using namespace std;
using ll = long long;

template<class T> inline T& cmin(T& a, const T& b) { if(b < a) a = b; return a; }
template<class T> inline T& cmax(T& a, const T& b) { if(a < b) a = b; return a; }
template<class... Args> void print(Args&&... args) {
    ((cout << args << ' '), ...);
}
template<class... Args> void println(Args&&... args) {
    print(args...), cout << endl;
}

const int N = 5e5 + 8, inf = 0x3f3f3f3f;

int n, h[N], d[N];

int solve() {
    vector<int> A, B;
    rep(i, 1, n) if(h[i] == 1) A.pb(i);
    rep(i, 1, n) if(d[i] == 1) B.pb(i);
    if(SZ(A) != SZ(B)) return inf;
    int res = 0;
    For(i, 0, SZ(A)) res += abs(A[i] - B[i]);
    return res;
}
int main() {
    cin.tie(0)->sync_with_stdio(0);
    cin >> n;
    rep(i, 1, n) cin >> h[i];
    for(int i = 2; i <= n; i += 2) h[i] = -h[i];
    int cnt = 0;
    rep(i, 1, n) cnt += h[i];
    int ans = inf;
    rep(k, 0, n) {
        rep(i, 1, k) d[i] = i & 1 ? -1 : 1;
        rep(i, k + 1, n) d[i] = i & 1 ? 1 : -1;
        cmin(ans, solve());
    }
    cout << (ans == inf ? -1 : ans) << '\n';
}