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;
long long n, ans, root;
long long ee, h[200005], nex[200005 << 1], to[200005 << 1];
void addedge(long long x, long long y) {
  nex[++ee] = h[x];
  to[ee] = y;
  h[x] = ee;
}
long long dfs(long long x, long long pre) {
  set<long long> s;
  for (long long i = h[x]; i; i = nex[i])
    if (to[i] != pre) {
      long long v = dfs(to[i], x);
      if (v == -1) return -1;
      s.insert(v + 1);
    }
  if (s.size() == 0) return 0;
  if (s.size() == 1) return *s.begin();
  if (s.size() == 2) {
    if (x == root)
      return *s.begin() + *--s.end();
    else
      root = x;
  }
  return -1;
}
signed main() {
  cin >> n;
  for (long long i = 1, x, y; i < n && cin >> x >> y; i++)
    addedge(x, y), addedge(y, x);
  root = 1;
  ans = dfs(root, 0);
  if (ans == -1 && root != 1) ans = dfs(root, 0);
  cout << ans << endl;
  return 0;
}
