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 root, n;
vector<int> mp[200605];
int Dfs(int u, int from) {
  set<int> s;
  for (int i = 0; i < mp[u].size(); i++) {
    int v = mp[u][i];
    if (v == from) continue;
    int tmp = Dfs(v, u);
    if (tmp == -1) return -1;
    s.insert(tmp + 1);
  }
  if (s.size() == 0) return 0;
  if (s.size() == 1) return *s.begin();
  if (s.size() == 2) {
    if (from == -1) {
      int sum = 0;
      for (set<int>::iterator it = s.begin(); it != s.end(); it++) {
        sum += *it;
      }
      return sum;
    } else {
      root = u;
      return -1;
    }
  }
  return -1;
}
int main() {
  while (~scanf("%d", &n)) {
    for (int i = 1; i <= n; i++) mp[i].clear();
    for (int i = 1; i <= n - 1; i++) {
      int x, y;
      scanf("%d%d", &x, &y);
      mp[x].push_back(y);
      mp[y].push_back(x);
    }
    root = 1;
    int ans = Dfs(root, -1);
    if (ans == -1 && root != 1) ans = Dfs(root, -1);
    printf("%d\n", ans);
  }
}
