#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 5e5;
vector<int> adj[MAX_N + 1];
vector<int> order;
void dfs( int u, int p ) {
if ( adj[u].size() == 1 )
order.push_back( u );
for ( int v: adj[u] ) {
if ( v == p )
continue;
dfs( v, u );
}
}
int main() {
cin.tie( nullptr );
ios_base::sync_with_stdio( false );
int n;
cin >> n;
for ( int i = 0; i < n - 1; i++ ) {
int u, v;
cin >> u >> v;
adj[u].push_back( v );
adj[v].push_back( u );
}
int r = 1;
while ( adj[r].size() == 1 )
r++;
dfs( r, 0 );
cout << (order.size() + 1) / 2 << "\n";
for ( int i = 0; i < order.size() / 2; i++ )
cout << order[i] << " " << order[order.size() - 1 - i] << "\n";
if ( order.size() % 2 == 1 )
cout << order[order.size() / 2] << " " << order[1] << "\n";
return 0;
}
