1 #include <iostream> 2 #include <vector> 3 #include <algorithm> 4 #include <map> 5 6 using namespace std; 7 8 int main(){ 9 int n; 10 cin >> n; 11 map<int,int> id; 12 for(int i = 0; i <n; i ++ ){ 13 int a; 14 cin >>a; 15 if(id.find(a) == id.end()) id[a] = 1; 16 else id[a]++; 17 } 18 int cnt = 0 ; 19 bool error = false; 20 for(map<int,int>::iterator it = id.begin(); it!=id.end(); it ++ ){ 21 if(it->first == 0) continue; 22 if(it->second > 2){error = true; break;} 23 if(it->second == 2) cnt++; 24 } 25 if(error) cout<<-1<<endl; 26 else if(cnt) cout<<cnt<<endl; 27 else cout<<0<<endl; 28 return 0; 29 }