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;
vector<int> E[200010];
bool visited[200010] = {0};
int flag = 0;
int depth = 0;
bool dfs(int u, int lev) {
  visited[u] = 1;
  int sz = E[u].size();
  if (sz == 1) {
    if (flag == 0) {
      flag = 1;
      depth = lev;
    }
    return (lev == depth);
  }
  for (int i = 0; i < sz; ++i)
    if (!visited[E[u][i]])
      if (dfs(E[u][i], lev + 1) == 0) return 0;
  return 1;
}
int main() {
  int n, u, v;
  cin >> n;
  for (int i = 1; i < n; ++i) {
    cin >> u >> v;
    E[u].push_back(v);
    E[v].push_back(u);
  }
  int root = 1;
  for (; root <= n; ++root)
    if (E[root].size() > 2) break;
  if (root == n + 1) {
    if (n % 2)
      cout << n / 2;
    else
      cout << n - 1;
    return 0;
  }
  visited[root] = 1;
  int sz = E[root].size(), cnt = 0, dis[3];
  for (int i = 0; i < sz; ++i) {
    flag = 0;
    if (dfs(E[root][i], 1) == 0) {
      cout << -1;
      return 0;
    }
    int j;
    for (j = 0; j < cnt; ++j) {
      if (dis[j] == depth) break;
    }
    if (j == cnt) dis[cnt++] = depth;
    if (cnt > 2) {
      cout << -1;
      return 0;
    }
  }
  int ans = 0;
  for (int i = 0; i < cnt; ++i) {
    ans += dis[i];
  }
  cout << ans;
  return 0;
}
