10
4 2
7 4
2 6
2 5
4 8
10 3
2 9
9 1
5 10
 10
5 10
7 8
8 3
2 6
3 2
9 7
4 5
10 1
6 4
 4
4 1
4 3
4 2
 3
3 1
1 2
 9
1 2
1 3
1 4
1 5
1 6
6 7
6 8
8 9
 5
1 5
2 3
2 4
1 2
 8
6 1
4 7
4 8
8 5
7 6
4 3
4 2
 7
5 6
5 7
5 1
7 4
6 3
3 2
 3
1 3
3 2
 2
1 2
 5
5 4
4 3
3 1
5 2
 6
1 6
3 1
6 4
5 3
2 5
 11
8 9
2 7
1 11
3 2
9 1
8 5
8 6
5 4
4 10
8 3
 11
11 9
6 7
7 1
8 11
5 6
3 5
9 3
10 8
2 4
4 10
 12
12 6
6 7
8 11
4 8
10 4
12 3
2 10
6 2
12 9
4 1
9 5

-1
 9
 1
 1
 3
 3
 -1
 -1
 1
 1
 1
 5
 1
 5
 -1

#include <bits/stdc++.h>
using namespace std;
const double PI = acos(-1.0);
template <class Int>
Int gcd(Int a, Int b) {
  return (b == 0 ? abs(a) : gcd(b, a % b));
}
const int dx[]{0, 0, 1, -1, 1, -1, 1, -1};
const int dy[]{1, -1, 0, 0, 1, -1, -1, 1};
const char dt[]{'R', 'L', 'D', 'U'};
void run() {}
const int N = 2e5 + 10, MOD = 998244353;
vector<int> adj[N];
vector<int> dp[N], dep;
int len;
void dfs(int u, int p) {
  dp[u][0] = p;
  for (int i = 1; i < len; i++) dp[u][i] = dp[dp[u][i - 1]][i - 1];
  for (auto v : adj[u])
    if (v != p) dep[v] = dep[u] + 1, dfs(v, u);
}
void build(int nodes, int root) {
  len = 1;
  while ((1 << len) <= nodes) len++;
  for (int i = 1; i <= nodes; i++) dp[i].resize(len + 1);
  dep = vector<int>(nodes + 1);
  dfs(root, root);
}
int lca(int u, int v) {
  if (u < v) swap(u, v);
  for (int k = len; k >= 0; k--)
    if (dep[u] - (1 << k) >= dep[v]) u = dp[u][k];
  if (u == v) return u;
  for (int k = len; k >= 0; k--)
    if (dp[u][k] != dp[v][k]) u = dp[u][k], v = dp[v][k];
  return dp[u][0];
}
int main() {
  cout << setprecision(10) << fixed;
  run();
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  for (int i = 1; i < n; i++) {
    int u, v;
    cin >> u >> v;
    adj[u].push_back(v);
    adj[v].push_back(u);
  }
  int cnt = 0;
  vector<int> nod;
  int root = -1;
  for (int i = 1; i <= n; i++)
    if (((int)(adj[i]).size()) == 1) {
      cnt++;
      nod.push_back(i);
    } else if (((int)(adj[i]).size()) == 3)
      root = i;
  if (cnt == 2) {
    if (n == 2)
      cout << "\n";
    else if ((n - 1) % 2 == 0)
      cout << n / 2 << "\n";
    else
      cout << -1 << "\n";
    return 0;
  }
  if (cnt > 3) return cout << -1 << "\n", 0;
  vector<int> lvl(n + 1);
  queue<int> q;
  q.push(root);
  lvl[root] = 1;
  while (((int)(q).size())) {
    int u = q.front();
    q.pop();
    for (auto v : adj[u]) {
      if (!lvl[v]) {
        lvl[v] = lvl[u] + 1;
        q.push(v);
      }
    }
  }
  set<int> st;
  for (int i = 0; i < 3; i++) st.insert(lvl[nod[i]]);
  if (((int)(st).size()) == 2) {
    int sum = 0;
    for (auto it : st) sum += it - 1;
    cout << sum << "\n";
  } else
    cout << -1 << "\n";
}
