#include <bits/stdc++.h>
using namespace std;
#define fast ios::sync_with_stdio(0);cin.tie(0);
typedef long long ll;
#define f first
#define s second
#define MAXN 100005
vector<vector<int>> graph;
int main() {
fast
int n;
cin >> n;
graph = vector<vector<int>>(n+1, vector<int>(0));
int deg[n+1];
memset(deg, 0, sizeof(deg));
for (int i = 0; i < n; i++) {
int u, v;
cin >> u >> v;
graph[u].push_back(v);
graph[v].push_back(u);
deg[u]++;
deg[v]++;
}
bool control = true;
for (int i = 1; i <= n; i++)
if (deg[i] != 2)
control = false;
if (control) {
if (n % 4 == 0)
cout << n / 2 << endl;
else
cout << -1 << endl;
}
}
