/*#pragma GCC optimize("O3")
#pragma GCC optimization("Ofast,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")*/
#include <bits/stdc++.h>
/*#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>*/
#define ll long long
#define ll1 long long
#define ull unsigned long long
#define dou long double
#define str string
#define vll vector<ll>
#define vi vector<int>
#define pll pair<ll, ll>
#define vpll vector<pair<ll, ll>>
#define vbool vector<bool>
#define vstr vector<str>
#define vvll vector<vll>
#define pb push_back
#define pf push_front
#define endl "\n"
#define fr first
#define se second
// #define sortcmp(a) sort(a.begin(), a.end(), cmp)
#define sort(a) sort(a.begin(), a.end())
#define reverse(a) reverse(a.begin(), a.end())
#define speed ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#define ordered_set tree<ll, null_type, less_equal<ll>, rb_tree_tag, tree_order_statistics_node_update>
using namespace std;
//using namespace __gnu_pbds;
const ll INF = 1e18+7;
const int lg = 20;
//const ll MOD = 1e9+7;
//const ll MOD2 = 998244353;
const ll MOD3 = 1e6+3;
mt19937 rng(1488);
ll randll(ll l, ll r) {
return uniform_int_distribution<ll>(l, r)(rng);
}
int main() {
speed;
ll n;
cin >> n;
vll a(n);
vpll v;
for (int i = 0; i < n; i ++) {
cin >> a[i];
if (v.empty() || v.back().fr != a[i]) v.pb({a[i], 1});
else v.back().se ++;
}
vpll dp(v.size()+7, {INF, INF});
dp[0] = {0, 0};
if (v[0].fr == 1) {
if (v[0].se % 2 == 0) dp[0].fr = v[0].se/2;
else dp[0].fr = INF;
}
if (v[0].fr == -1) {
if (v[0].se % 2 == 0) dp[0].se = v[0].se/2;
else dp[0].se = INF;
}
for (int i = 1; i < v.size(); i ++) {
if (v[i].fr == -1) {
dp[i].fr = dp[i-1].fr;
if (v[i].se % 2 == 0) dp[i].se = min(dp[i-1].fr, dp[i-1].se)+v[i].se/2;
else dp[i].se = INF;
}
else {
dp[i].se = min(dp[i-1].fr, dp[i-1].se);
if (v[i].se % 2 == 0) dp[i].fr = dp[i-1].fr+v[i].se/2;
else dp[i].fr = INF;
}
}
/*for (auto i = 0; i < v.size(); i ++) {
cout << dp[i].fr << ' ' << dp[i].se << endl;
}*/
if (min(dp[v.size()-1].fr, dp[v.size()-1].se) >= INF) cout << -1 << endl;
else cout << min(dp[v.size()-1].fr, dp[v.size()-1].se) << endl;
}
