#include<bits/stdc++.h>
using namespace std ;
typedef long long ll ;
typedef unsigned long long ull ;
#define fi first
#define se second
// mt19937 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
const int MAXN = 5e5 + 7 ;
int n ;
vector < int > v[ MAXN ] , ord ;
int cnt[ MAXN ] , root , prv[ MAXN ] ;
int spec ;
void init ( int x , int lst ) {
bool leaf = true ;
cnt[ x ] = 0 ;
for ( auto y : v[ x ] ) {
if ( y == lst ) { continue ; }
leaf = false ;
prv[ y ] = x ;
init ( y , x ) ;
cnt[ x ] += cnt[ y ] ;
}
if ( leaf == true ) {
++ cnt[ x ] ;
}
}
int get_centroid ( int x , int lst , int targ ) {
for ( auto y : v[ x ] ) {
if ( y == lst ) { continue ; }
if ( cnt[ y ] >= targ ) {
return get_centroid ( y , x , targ ) ;
}
}
return x ;
}
void dfs ( int x , int lst ) {
bool leaf = true ;
for ( auto y : v[ x ] ) {
if ( y == lst ) { continue ; }
leaf = false ;
dfs ( y , x ) ;
}
if ( leaf == true && cnt[ x ] > 0 ) {
ord.push_back ( x ) ;
}
}
vector < pair < int , int > > ans ;
void solve ( ) {
cin >> n ;
for ( int i = 1 , x , y ; i < n ; ++ i ) {
cin >> x >> y ;
v[ x ].push_back ( y ) ;
v[ y ].push_back ( x ) ;
}
for ( int i = 1 ; i <= n ; ++ i ) {
if ( ( int ) v[ i ].size ( ) >= 2 ) {
root = i ; break ;
}
}
init ( root , -1 ) ;
root = get_centroid ( root , -1 , cnt[ root ] / 2 + 1 ) ;
init ( root , -1 ) ;
dfs ( root , -1 ) ;
for ( int i = 0 ; i < cnt[ root ] / 2 ; ++ i ) {
ans.push_back ( { ord[ i ] , ord[ i + cnt[ root ] / 2 ] } ) ;
}
if ( ( cnt[ root ] % 2 ) == 1 ) {
ans.push_back ( { root , ord.back ( ) } ) ;
}
int sz = ans.size ( ) ;
cout << sz << "\n" ;
for ( auto [ x , y ] : ans ) {
cout << x << " " << y << "\n" ;
}
}
int main ( ) {
ios_base :: sync_with_stdio ( false ) ;
cin.tie ( NULL ) ;
int t = 1 ; // cin >> t ;
while ( t -- ) { solve ( ) ; }
return 0 ;
}
