#include <iostream> #include <cstring> #include <vector> using namespace std; int N; vector<int> adjl[100005]; bool visited[100005]; int see[100005]; int main(){ cin >> N; for(int i = 0; i < N; i++){ int a, b; cin >> a >> b; adjl[a].push_back(b); adjl[b].push_back(a); } memset(visited, false, sizeof(visited)); memset(see, 0, sizeof(see)); if(N % 4) cout << -1; else cout << N /2; }