#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++ << ": ";
_();
}
}
