CROC 2013 A. Spyke Talks

 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 }