#include <bits/stdc++.h>
using namespace std;
#define print(l) for(auto i:l) cout<<i<<" ";cout<<endl;
#define input(t,l,n) vector<t>l(n);for(int i = 0;i<n;i++)cin>>l[i];
#define int long long
#define pb push_back
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
#define all(l) l.begin(),l.end()
#define pii pair<int,int>
#define fi first
#define se second
const int M = 1e9+7;
const int inf = 1e18;
int bp(int x, int y, int p){
int res = 1;
x = x % p;
while (y > 0) {
if (y & 1)
res = (res * x) % p;
y = y >> 1;
x = (x * x) % p;
}
return res;
}
int MI(int n, int p){
return bp(n, p - 2, p);
}
int mul(int x,int y, int p){
return x * 1ull * y % p;
}
int di(int x,int y, int p){
return mul(x, MI(y, p), p);
}
int n , m , k , q;
void solve(int testcase_number){
cin>>n;
input(int,l,n);
vector<vector<int>>comps;
int f = l[0];
int i = 0;
vector<int>t;
while(i<n){
while(l[i] == f){
t.pb(l[i]);
i++;
}
f = l[i];
comps.pb(t);
t = {};
}
int ne = 1;
int po = 1;
int toup = 1;
int ans1 = 0;
int fne = 0;
int ans2 = 0;
for(auto i:comps){
// print(i);
if(i[0] == -1){
po = toup;
if(fne == 0)continue;
if(i.size()%2 !=0){
ne = 0;
}
else{
ans1+=(i.size()/2);
}
}
else{
fne = 1;
if(i.size()%2!=0){
toup = 0;
}
else{
ans2+=(i.size()/2);
}
}
}
// cout<<ne<<" "<<po<<endl;
if(ne == 0 and po == 0){
cout<<-1<<endl;
}
else if(ne == 0 and po == 1){
cout<<ans2<<endl;
}
else if(ne == 1 and po == 0){
cout<<ans1<<endl;
}
else{
cout<<min(ans1,ans2)<<endl;
}
}
signed main(){
ios::sync_with_stdio(0);//DO NOT USE IN INTERACTIVE
cin.tie(0), cout.tie(0);
cout << fixed<<setprecision(9);
int t = 1;
// cin>>t;
for(int i = 1;i<=t;i++){
solve(i);
}
}
