#include <bits/stdc++.h>
using namespace std;
const int N = 5e5+5;
int t, n, a[N], adj[N];
pair<int, int> b[N];
deque<int> q[2][N];
bitset<N> vis;
void dfs(int x) {
vis[x] = 1;
if (adj[x] != -1 && !vis[adj[x]]) dfs(adj[x]);
}
signed main() {
ios::sync_with_stdio(false); cin.tie(nullptr);
cin >> n;
for (int i = 0; i < n; i++) {
adj[i] = -1;
cin >> a[i];
b[i] = make_pair(abs(a[i]), i);
}
sort(b, b+n);
int idx = 0;
for (int i = 0; i < n; i++) {
if (i && b[i].first > b[i-1].first) idx++;
int x = b[i].second;
if (a[x] < 0) q[0][idx].push_back(x);
else q[1][idx].push_back(x);
}
int L = 0;
int R = n-1;
while (idx >= 0) {
while (!q[0][idx].empty() && q[0][idx].front() == L) {
q[0][idx].pop_front();
L++;
}
while (!q[1][idx].empty() && q[1][idx].back() == R) {
q[1][idx].pop_back();
R--;
}
while (!q[0][idx].empty()) {
adj[q[0][idx].back()] = R;
q[0][idx].pop_back();
R--;
}
while (!q[1][idx].empty()) {
adj[q[1][idx].front()] = L;
q[1][idx].pop_front();
L++;
}
idx--;
}
for (int i = 0; i < n; i++) {
if (adj[i] == i) {
cout << "-1\n";
return 0;
}
}
int ans = n;
for (int i = 0; i < n; i++) {
if (vis[i]) continue;
ans--;
dfs(i);
}
cout << ans << "\n";
}
