#include <bits/stdc++.h>
#define fastio ios_base::sync_with_stdio(0); cin.tie(0);
#define int long long
using namespace std;
const int N = 1e5+5;
vector<int> adj[N];
int color[N];
signed main(){
fastio
int n;
cin >> n;
for(int i = 1;i <= n;i++){
int u,v;
cin >> u >> v;
u--,v--;
adj[u].push_back(v);
adj[v].push_back(u);
}
if(n <= 20){
int mn = 1e18;
for(int i = 0; i < (1<<n); i++){
bool ok = true;
for(int j = 0; j < n; j++){
color[j] = 0;
}
for(int j = 0; j < n; j++){
if(i & (1<<j))
color[j] = 1;
}
for(int j = 0; j < n; j++){
int cnt = 0;
for(auto v : adj[j]){
if(color[v]) cnt++;
}
if(cnt != 1) ok = false;
}
if(ok) mn = min(mn, (int)__builtin_popcount(i));
}
cout << (mn==1e18 ? -1 : mn) << "\n";
}else{
cout << (n%4==0 ? 2*n/4 : -1) << "\n";
}
}
