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 n, d[10002], ans, prv[10002], path[10002];
int u, v;
vector<int> V[10002];
queue<int> Q;
void read() {
  scanf("%d", &n);
  for (int i = 1; i < n; ++i) {
    int x, y;
    scanf("%d %d", &x, &y);
    V[x].push_back(y);
    V[y].push_back(x);
  }
}
void bfs(int &x) {
  int i, X = x;
  memset(d, 0, sizeof(d));
  Q.push(x);
  while (!Q.empty()) {
    int nod, y = Q.front();
    Q.pop();
    for (i = 0; i < V[y].size(); ++i)
      if ((!d[nod = V[y][i]] || d[nod] > d[y] + 1) && nod != X) {
        d[nod] = d[y] + 1;
        prv[nod] = x;
        Q.push(nod);
        if (d[nod] > d[x]) x = nod;
      }
  }
}
void solve() {
  int i, j = 0, k = 0;
  u = 1;
  bfs(u);
  v = u;
  bfs(v);
  for (i = v; i != u; i = prv[i]) path[++j] = i;
  path[++j] = u;
  ans = j;
}
void write() {
  int i;
  ++ans;
  if (ans & 1)
    printf("-1\n");
  else
    printf("%d\n", ans / 2);
}
int main() {
  read();
  solve();
  write();
  return 0;
}
