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;
template <class T>
T pmax(T a, T b) {
  return max(a, b);
}
const long long int mod = 1e9 + 7;
const long long int Maa = 2e5 + 3;
vector<long long int> edge[Maa];
long long int n, m;
long long int act_dfs(long long int V, long long int P, long long int l) {
  int e = 0, val = 0;
  vector<long long int> v;
  for (auto u : edge[V]) {
    if (u == P) continue;
    val = 1 + act_dfs(u, V, l);
    e++;
    v.push_back(val);
  }
  if (V != l and e >= 2) {
    cout << -1;
    exit(0);
  }
  if (V == l) {
    sort(v.begin(), v.end());
    reverse(v.begin(), v.end());
    if (v[0] == v[1])
      cout << v[0] + v[2];
    else if (v[1] == v[2])
      cout << v[0] + v[1];
    else
      cout << -1;
  }
  return val;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int T, l, r, f = 0;
  cin >> n;
  vector<long long int> v;
  for (int i = 0; i < n - 1; i++) {
    cin >> l >> r;
    edge[l].push_back(r);
    edge[r].push_back(l);
    if (edge[l].size() >= 3) v.push_back(l);
    if (edge[r].size() >= 3) v.push_back(r);
  }
  if (v.size() > 1)
    cout << -1;
  else if (v.size() == 1 and edge[v[0]].size() > 3)
    cout << -1;
  else if (v.size() == 1)
    act_dfs(v[0], 0, v[0]);
  else if (n % 2 != 0)
    cout << n / 2;
  else
    cout << n;
}
