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;
void check(bool x) {
  if (!x) {
    cout << "-1\n";
    exit(0);
  }
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  vector<set<int>> g(n + 1);
  for (int i = 0; i < int(n - 1); i++) {
    int a, b;
    cin >> a >> b;
    g[a].insert(b);
    g[b].insert(a);
  }
  bool path = 1;
  for (int i = 1; i <= int(n); i++) {
    if ((int)g[i].size() > 2) {
      path = 0;
    }
  }
  if (path) {
    cout << n - 1 << "\n";
    exit(0);
  }
  queue<pair<int, int>> leaves;
  for (int i = 1; i <= int(n); i++) {
    if ((int)g[i].size() == 1) {
      leaves.emplace(i, 0);
    }
  }
  vector<set<int>> lengths(n + 1);
  while (!leaves.empty()) {
    auto [u, len] = leaves.front();
    leaves.pop();
    assert((int)g[u].size() == 1);
    int v = *g[u].begin();
    g[v].erase(u);
    lengths[v].insert(len + 1);
    if ((int)g[v].size() == 1 && (int)lengths[v].size() == 1) {
      lengths[v].clear();
      leaves.emplace(v, len + 1);
    }
  }
  int guy = -1;
  for (int i = 1; i <= int(n); i++) {
    if (!lengths[i].empty()) {
      check(guy == -1);
      check((int)lengths[i].size() < 3);
      guy = i;
    }
  }
  assert(guy != -1);
  int s = 0;
  for (int x : lengths[guy]) {
    s += x;
  }
  cout << s << endl;
}
