解：

#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
vector<vector<int>>edges; 
vector<int>parent;        
int cut_count=0;       
int dfs(int u){
    int size = 1; 
    for (int v : edges[u]) {
        if (v != parent[u]) {
            parent[v] = u; 
            size += dfs(v); 
        }
    }
    if (size == 3) { 
        cut_count++;
        return 0;  
    }
    return size;
}
int main() {
    int n;
    cin >> n;
    if (n % 3 != 0) { 
        cout << -1 << endl;
        return 0;
    }
    edges.resize(n + 1);
    parent.resize(n + 1, -1);
    for (int i = 0; i < n - 1; i++) {
        int u, v;
        cin >> u >> v;
        edges[u].push_back(v);
        edges[v].push_back(u);
    }
    if (dfs(1) != 0) {
        cout << -1 << endl;
    } else { 
        cout << cut_count - 1 << endl;
    }
    return 0;
}

欢迎参考
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