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

/*
 * 765E. Tree Folding.cpp
 *
 *  Created on: Feb 15, 2017
 *      Author: Osama Hatem
 */

#include <bits/stdtr1c++.h>
#include <ext/numeric>

using namespace std;

const int MAXN = 2e5 + 5;

vector<int> adj[MAXN];
int n, deg[MAXN];
set<int> dist[MAXN];

void NO() {
	printf("-1\n");
	exit(0);
}

int main() {
#ifndef ONLINE_JUDGE
	freopen("in.in", "r", stdin);
//	freopen("out.out", "w", stdout);
#endif

	scanf("%d", &n);
	for (int i = 1; i < n; i++) {
		int u, v;
		scanf("%d%d", &u, &v);
		adj[u].push_back(v), adj[v].push_back(u);
		deg[u]++, deg[v]++;
	}
	queue<int> Q;
	for (int i = 1; i <= n; i++)
		if (deg[i] == 1)
			Q.push(i), dist[i].insert(0);
	int ans = 0;
	while (Q.size()) {
		int x = Q.front();
		Q.pop();
		if (!deg[x] && dist[x].size() < 3)
			for (auto it = dist[x].begin(); it != dist[x].end(); it++)
				ans += *it;
		else if (dist[x].size() > 1)
			NO();
		for (int y : adj[x]) {
			if (!deg[y])
				continue;
			deg[x]--, deg[y]--;
			dist[y].insert(*dist[x].begin() + 1);
			if (deg[y] == 1)
				Q.push(y);
		}
	}
	printf("%d\n", ans);
	return 0;
}
