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 powmod(long long a, long long b, long long p) {
  long long res = 1;
  a %= p;
  for (; b; b >>= 1) {
    if (b & 1) res = res * a % p;
    a = a * a % p;
  }
  return res;
}
template <typename T>
T gcd(T x, T y) {
  while (y) {
    T t = x % y;
    x = y;
    y = t;
  }
  return x;
}
template <typename T>
T lsM(T x, T y) {
  return x / gcd(x, y) * y;
}
template <typename T>
inline bool chkmin(T &a, const T &b) {
  return a > b ? a = b, 1 : 0;
}
template <typename T>
inline bool chkmax(T &a, const T &b) {
  return a < b ? a = b, 1 : 0;
}
int fst[220000], nxt[400100], vv[400100], e;
int vis[220000], deg[220000];
set<int> s[220000];
int n;
void init() {
  memset(fst, -1, sizeof fst);
  e = 0;
}
void add(int u, int v) {
  vv[e] = v, nxt[e] = fst[u], fst[u] = e++;
  vv[e] = u, nxt[e] = fst[v], fst[v] = e++;
}
int main() {
  init();
  scanf("%d", &n);
  for (int i = 1; i < n; i++) {
    int u, v;
    scanf("%d%d", &u, &v);
    add(u, v);
    deg[u]++, deg[v]++;
  }
  queue<int> q;
  for (int i = 1; i <= n; i++)
    if (deg[i] == 1) {
      q.push(i);
      s[i].insert(0);
    }
  while (!q.empty()) {
    int u = q.front();
    q.pop();
    vis[u] = 1;
    for (int i = fst[u]; ~i; i = nxt[i]) {
      int v = vv[i];
      if (vis[v]) continue;
      deg[v]--;
      s[v].insert(*(s[u].begin()) + 1);
      if (deg[v] == 1 && (int)s[v].size() == 1) q.push(v);
    }
  }
  int cnt = 0, ans = 0;
  for (int i = 1; i <= n; i++) {
    if ((int)s[i].size() > 2) {
      puts("-1");
      return 0;
    } else if ((int)s[i].size() == 1) {
      ans = max(ans, *(s[i].begin()));
    } else {
      cnt++;
      set<int>::iterator it = s[i].end();
      it--;
      ans = *(s[i].begin()) + *it;
    }
    if (cnt > 1) {
      puts("-1");
      return 0;
    }
  }
  printf("%d\n", ans % 2 == 0 ? ans / 2 : ans);
  return 0;
}
