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>
const int maxn = 2e5 + 10;
const int N = 5e4 + 10;
const int M = N * 10 * 10;
using namespace std;
inline long long gcd(long long p, long long q) {
  return q == 0 ? p : gcd(q, p % q);
}
inline long long qpow(long long p, long long q) {
  long long f = 1;
  while (q) {
    if (q & 1) f = f * p;
    p = p * p;
    q >>= 1;
  }
  return f;
}
inline void umax(long long &p, long long q) {
  if (p < q) p = q;
}
inline void umin(long long &p, long long q) {
  if (p > q) p = q;
}
inline long long read() {
  long long x = 0;
  int f = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}
int n, m, k, t;
vector<int> e[maxn], vec[maxn];
bool flag = true;
void dfs(int x, int y, int z) {
  if (!flag) return;
  for (int k : e[x]) {
    if (k == y) continue;
    dfs(k, x, z);
    for (int t : vec[k]) vec[x].push_back(t + 1);
  }
  if ((int)vec[x].size() == 0)
    vec[x].push_back(0);
  else {
    sort(vec[x].begin(), vec[x].end());
    vec[x].erase(unique(vec[x].begin(), vec[x].end()), vec[x].end());
    if (x != z && vec[x].size() > 1) flag = false;
  }
}
int main() {
  int i, j;
  scanf("%d", &n);
  for (i = 1; i <= n - 1; i++)
    scanf("%d%d", &j, &k), e[j].push_back(k), e[k].push_back(j);
  for (i = 1; i <= n; i++)
    if ((int)e[i].size() >= 3) break;
  if (i == n + 1) return 0 * printf("%d\n", n - 1);
  dfs(i, -1, i);
  if (!flag)
    puts("-1");
  else if ((int)vec[i].size() > 2)
    return 0 * puts("-1");
  else
    printf("%d\n", vec[i].size() == 1 ? 2 * vec[i][0] : vec[i][0] + vec[i][1]);
  return 0;
}
