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;
const int N = 2e5 + 5;
int n, dep[N], fa[N];
vector<int> e[N];
void dfs(int u, int f, int d) {
  fa[u] = f;
  dep[u] = d;
  for (auto v : e[u])
    if (v != f) dfs(v, u, d + 1);
}
int getmi() {
  dfs(1, 0, 1);
  int u1 = 1;
  for (int i = 2; i <= n; ++i)
    if (dep[u1] < dep[i]) u1 = i;
  dfs(u1, 0, 1);
  int u2 = 1;
  for (int i = 2; i <= n; ++i)
    if (dep[u2] < dep[i]) u2 = i;
  int p = u2;
  while (dep[p] != dep[u1] + dep[u2] >> 1) p = fa[p];
  return p;
}
int solve(int u, int f) {
  int re = 0, re2 = 0;
  for (auto v : e[u])
    if (v != f) {
      int t = solve(v, u);
      if (!re)
        re = t;
      else if (re != t)
        if (!f && !re2)
          re2 = t;
        else
          cout << -1, exit(0);
    }
  if (!f) return re + re2;
  return re + 1;
}
int main() {
  cin >> n;
  for (int i = 1; i <= n - 1; ++i) {
    int x, y;
    cin >> x >> y;
    e[x].push_back(y);
    e[y].push_back(x);
  }
  cout << solve(getmi(), 0);
}
