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;
const int maxn = 2e5 + 10;
int head[maxn], cnt, fa[maxn];
struct edge {
  int to, nxt;
} e[maxn * 2];
int n, st, ed, ml;
int num, ans, tag;
set<int> s[maxn];
void add(int x, int y) {
  e[++cnt] = (edge){y, head[x]};
  head[x] = cnt;
}
void dfs(int x, int d) {
  if (d > ml) {
    ml = d;
    ed = x;
  }
  for (int i = head[x]; i; i = e[i].nxt) {
    int u = e[i].to;
    if (u == fa[x]) continue;
    fa[u] = x;
    dfs(u, d + 1);
  }
}
void dfs_ans(int x, int f) {
  int opt = 0;
  for (int i = head[x]; i; i = e[i].nxt) {
    int u = e[i].to;
    if (u == f) continue;
    dfs_ans(u, x);
    opt++;
    for (set<int>::iterator it = s[u].begin(); it != s[u].end(); ++it)
      s[x].insert(*it + 1);
  }
  if (!opt) s[x].insert(1);
  if (s[x].size() == 2) {
    num++;
    for (set<int>::iterator it = s[x].begin(); it != s[x].end(); ++it)
      ans += *it;
    ans -= 2;
  } else if (s[x].size() > 2)
    tag = 1;
}
int main() {
  ios::sync_with_stdio(false);
  cin >> n;
  for (int i = 1; i < n; i++) {
    int x, y;
    cin >> x >> y;
    add(x, y);
    add(y, x);
  }
  ml = 0;
  st = 1;
  dfs(st, 0);
  ml = 0;
  st = ed;
  fa[st] = 0;
  dfs(st, 0);
  int cn = 0;
  while (cn < ml / 2) {
    cn++;
    ed = fa[ed];
  }
  dfs_ans(ed, 0);
  if (num > 1 || tag)
    cout << -1 << endl;
  else
    cout << ans << endl;
  return 0;
}
