#include <bits/stdc++.h>
using namespace std;
#ifdef Ramzi
#include "debug.h"
#else
#define debug(...)
#endif
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n;
cin >> n;
vector<vector<int>> g(n);
vector<int> deg(n);
for (int i = 0; i < n; i++) {
int a, b;
cin >> a >> b;
--a, --b;
++deg[a], ++deg[b];
g[a].push_back(b);
g[b].push_back(a);
}
vector<int> order(n);
iota(order.begin(), order.end(), 0);
sort(order.begin(), order.end(), [&](int i, int j) {
return (deg[i] > deg[j]);
});
int ans = 0;
vector<bool> seen(n);
bool fail = false;
auto Apply = [&](int u) {
vector<bool> f(n);
bool ok = false;
for (int v : g[u]) {
f[v] = true;
ok |= seen[v];
}
if (!ok) {
ans += 1;
for (int v : g[u]) {
seen[v] = true;
}
}
int mx = 0, node = -1;
for (int v : g[u]) {
bool found = false;
for (int x : g[v]) {
found |= f[x];
found |= seen[x];
}
if (!found && deg[v] > mx) {
mx = deg[v];
node = v;
}
}
if (node == -1) {
fail = true;
return;
}
for (int v : g[node]) {
seen[v] = true;
}
ans += 1;
};
for (int i : order) {
if (!seen[i]) {
Apply(i);
}
}
cout << (fail ? -1 : ans) << "\n";
return 0;
}
