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>
struct Edge {
  int from, to, next;
} f[100010 * 2];
bool Q = true;
int n, l = 0, head[100010], c[100010], root, w[100010], u, v, dg[100010], m,
       cnt = 0;
int Dfs(int k, int fa) {
  int t = 0;
  for (int i = head[k]; i; i = f[i].next)
    if (f[i].to != fa) t = w[f[i].to] = Dfs(f[i].to, k);
  for (int i = head[k]; i; i = f[i].next)
    if (f[i].to != fa && w[f[i].to] != t) Q = false;
  return t + 1;
}
int main() {
  scanf("%d", &n);
  for (int i = 1; i < n; i++) {
    scanf("%d%d", &u, &v);
    ({
      f[++l] = (Edge){u, v, head[u]};
      head[u] = l;
    });
    ({
      f[++l] = (Edge){v, u, head[v]};
      head[v] = l;
    });
    dg[u]++;
    dg[v]++;
  }
  for (int i = 1; i <= n; i++)
    if (dg[i] > 2) {
      root = i;
      break;
    }
  for (int i = head[root]; i; i = f[i].next)
    c[++m] = w[f[i].to] = Dfs(f[i].to, root);
  std::sort(c + 1, c + m + 1);
  c[0] = c[1] - 1;
  for (int i = 1; i <= m; i++)
    if (c[i] != c[i - 1]) cnt++;
  if (cnt > 2) Q = false;
  if (!Q)
    printf("-1\n");
  else
    printf("%d\n", c[1] + c[m]);
  return 0;
}
