This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include <bits/stdc++.h>
using namespace std;
#define all(x) x.begin(),x.end()
#define ll long long
#define LCBorz ios_base::sync_with_stdio(false);cin.tie(0);
const int N=1000005;
const int mod=1e6+3;
const int INF=1e9;
int32_t main(){
LCBorz;
int n;cin>>n;
vector<int> v(n+2),pre(n+2),suf(n+2);
for(int i=1;i<=n;i++){
cin>>v[i];
}
int last=0,cnt=0,can=1;
for(int i=1;i<=n;i++){
if(v[i]==-1){
if(last)can=0;
}
else{
cnt++;
last^=1;
}
if(can&&last==0){
pre[i]=cnt/2;
}
else{
pre[i]=INF;
}
}
last=0,can=1,cnt=0;
for(int i=n;i>=1;i--){
if(v[i]==1){
if(last)can=0;
}
else{
cnt++;
last^=1;
}
if(can&&last==0){
suf[i]=cnt/2;
}
else{
suf[i]=INF;
}
}
int ans=INF;
for(int i=0;i<=n;i++){
ans=min(ans,pre[i]+suf[i+1]);
}
if(ans==INF){
cout<<-1<<endl;
}
else{
cout<<ans<<endl;
}
return 0;
}
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