#include "bits/stdc++.h" using namespace std; #define intt long long #define fi first #define se second const intt mxN = 1e5 + 5; const intt LG = 20; const intt inf = 1e18; intt n,c; vector<vector<intt>> graph; vector<intt> color(mxN); void dfs(intt node) { color[node] = 1; for(auto u : graph[node]) { if(color[u] == 0) { dfs(u); } else if(color[u] == 1) { c=1; return; } } color[node] = 2; } vector<intt> boo; void dfs2(intt node) { color[node]=1; for(auto u : graph[node]) { if(!color[u]) { dfs2(u); } } boo.push_back(node); } void _() { cin >> n; graph.assign(n + 1, vector<intt> {}); color.assign(n + 1, 0ll); for(intt i = 1; i <= n - 1; i++) { intt a, b; cin >> a >> b; graph[a].push_back(b); } for(intt i = 1; i <= n; i++) { if(!color[i]) { dfs(i); if(c)break; } } if(c) { cout << -1 << endl; return; } color.assign(n + 1, 0ll); for(intt i = 1; i <= n; i++) { if(!color[i]) { dfs2(i); } } cout << boo.size() << endl; for(intt i = 0; i < boo.size(); i++) { cout << boo[i] << " 0" << endl; } } int main() { ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL); intt t = 1, buu = 1; // cin >> t; while(t--){ // cout << "Case #" << buu++ << ": "; _(); } }