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;
int main() {
  int n = 0;
  int ans = 0;
  vector<vector<int> > adj(n);
  for (int i = 0; i < n - 1; i++) {
    int u = 0, v = 0;
    cin >> u >> v;
    u--;
    v--;
    adj[u].push_back(v);
    adj[v].push_back(u);
  }
  queue<int> q;
  vector<int> l(n);
  vector<int> used(n);
  for (int i = 0; i < n; i++)
    if (adj.size() == 1) {
      q.push(i);
      l[i] = 1;
      used[i] = 1;
    }
  while (!q.empty()) {
    int curr = q.front();
    q.pop();
    for (int j = 0; j < adj[curr].size(); j++) {
      int v = adj[curr][j];
      used[v]++;
      if (used[v] == adj[v].size() - 1) {
        q.push(v);
      }
    }
    if (adj[curr].size() > 1) {
      if (used[curr] == adj[curr].size() - 1) {
        int v = adj[curr][0];
        int k = 0, idx = -1;
        if (l[v] != 0)
          k = l[v];
        else
          k = l[adj[curr][1]];
        for (int j = 0; j < adj[curr].size(); j++) {
          v = adj[curr][j];
          if (l[v] != k) {
            if (idx == -1 && l[v] == 0) {
              idx = v;
            } else {
              cout << "-1";
              return (0);
            }
          }
        }
        l[idx] = k + 1;
        ans = k + 1;
      }
    }
  }
  for (int i = 0; i < n; i++)
    if (l[i] == 0) {
      cout << "-1";
      return (0);
    }
  cout << ans;
}
