#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define MOD 1000003
const int N = 500005;
ll n, h[N], s1[N], s2[N], f[N], g[N], ans = 1e18;
void solve()
{
cin >> n;
for (int i = 1; i <= n; i++)
{
cin >> h[i];
}
for (int i = 0; i <= n + 1; i++)
{
f[i] = g[i] = 1e18;
}
f[0] = 0;
for (int i = 1; i <= n; i++)
{
f[i] = f[i - 1];
s1[i] = s1[i - 1];
if (h[i] == 1)
{
if (i & 1)
{
s1[i]++;
if (s1[i - 1] >= 0)
{
f[i] -= i;
}
else
{
f[i] += i;
}
}
else
{
s1[i]--;
if (s1[i - 1] <= 0)
{
f[i] -= i;
}
else
{
f[i] += i;
}
}
}
}
g[n + 1] = 0;
for (int i = n; i >= 1; i--)
{
g[i] = g[i + 1];
s2[i] = s2[i + 1];
if (h[i] == -1)
{
if (i & 1)
{
s2[i]++;
if (s2[i + 1] >= 0)
{
g[i] += i;
}
else
{
g[i] -= i;
}
}
else
{
s2[i]--;
if (s2[i + 1] <= 0)
{
g[i] += i;
}
else
{
g[i] -= i;
}
}
}
}
for (int i = 0; i <= n; i++)
{
if (s1[i] == 0 and s2[i + 1] == 0)
{
ans = min(ans, f[i] + g[i + 1]);
}
}
if (ans == 1e18)
{
ans = -1;
}
cout << ans << "\n";
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
ll t = 1;
// cin >> t;
for (int i = 1; i <= t; i++)
{
solve();
}
}
