#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];
pair<int, int> a[N];
ll dp[N][2];

ll init() {
    ll res = 0;
    rep(i, 1, n) {
        auto [v, p] = a[i];
        if(i & 1 ? v < 0 : v > 0) return inf;
        For(j, 1, i) res += a[j].second > p;
    }
    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];
    rep(i, 1, n) a[i] = {h[i], i};
    sort(a + 1, a + n + 1, [](auto a, auto b) {
        return abs(a.first) < abs(b.first);
    });
    ll ans = init();
    rep(i, 1, n) {
        auto [v, p] = a[i];
        ll L = 0, R = 0;
        For(j, 1, i) {
            L += a[j].second < p;
            R += a[j].second > p;
        }
        bool flag[2] = {v <= 0, v >= 0};
        dp[i][0] = dp[i][1] = inf;
        rep(j, 0, 1) {
            if(flag[j]) cmin(dp[i][!j], dp[i - 1][j] + L);
            if(flag[j ^ (i & 1)]) cmin(dp[i][j], dp[i - 1][j] + R);
        }
    }
    cmin(ans, dp[n][1]);
    cout << (ans == inf ? -1 : ans) << '\n';
}