#include <bits/stdc++.h>
//#include <ext/pb_ds/assoc_container.hpp>
#define INF 1000000021
#define MOD 1000000007
#define pb push_back
#define sqr(a) (a)*(a)
#define M(a, b) make_pair(a,b)
#define T(a, b, c) make_pair(a, make_pair(b, c))
#define F first
#define S second
#define all(x) (x.begin(), x.end())
#define deb(x) cerr << #x << " = " << x << '\n'
#define N 522222
using namespace std;
//using namespace __gnu_pbds;
typedef long double ld;
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<ll> vll;
//typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> indexed_set;
const ld pi = 2 * acos(0.0);
template<class T> bool umin(T& a, T b){if(a>b){a=b;return 1;}return 0;}
template<class T> bool umax(T& a, T b){if(a<b){a=b;return 1;}return 0;}
template<class T, class TT> bool pal(T a, TT n){int k=0;for(int i=0;i<=n/2;i++){if(a[i]!=a[n-i-1]){k=1;break;}}return k?0:1;}
//int month[] = {0,31,28,31,30,31,30,31,31,30,31,30,31};
vi v[N], l;
void dfs(int u, int e)
{
if(v[u].size() == 1)
l.pb(u);
for(auto it : v[u])
if(it != e)
dfs(it, u);
}
int main()
{
int n;
cin >> n;
for(int i = 2; i <= n; i ++)
{
int x, y;
cin >> x >> y;
v[x].pb(y);
v[y].pb(x);
}
dfs(1, 0);
cout << (l.size() + 1) / 2 << '\n';
for(int i = 0; i < (l.size() + 1) / 2; i ++)
cout << l[i] << ' ' << l[i + l.size()/2] << '\n';
getchar();
getchar();
return 0;
//ios::sync_with_stdio(false);
//cin.tie(0);
}
