#include <bits/stdc++.h>
using namespace std;
vector<int> dfs(int u, int p, const vector<vector<int>> &g, vector<pair<int,int>> &pairs) {
	// TODO: fix calling this from the root node.
	if(g[u].size() == 1)
		return {u};
	vector<int> ret, last;
	for(auto w : g[u])
		if(w != p){
			auto v = dfs(w, u, g, pairs);
			if(ret.size() == 0){
				ret.push_back(v.back());
				v.pop_back();
			}
			while(!last.empty() && !v.empty()){
				pairs.push_back({last.back(), v.back()});
				last.pop_back();
				v.pop_back();
			}
			for(auto x : v){
				last.push_back(x);
			}
		}

	for(auto x : last) ret.push_back(x);

	assert(ret.size() <= 3);
	if(ret.size() == 3){
		pairs.push_back({ret[0], ret[2]});
		ret[0] = ret[1];
		ret.resize(1);
	}
	assert(ret.size() > 0 && ret.size() <= 2);
	if(p == -1){
		if(ret.size() == 2)
			pairs.push_back({ret[0], ret[1]});
		else if(ret.size() == 1){
			pairs.push_back({ret[0], pairs.back().second});
		}
	}
	return ret;
}
int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	cout.tie(NULL);

	int n, p;
	cin >> n >> p;
	if(n == 2){
		cout << "1\n0 1\n";
		return 0;
	}
	vector<vector<int>> g(n);
	vector<pair<int,int>> pairs;
	for(int i = 0; i < n - 1; ++i) {
		int u, v;
		cin >> u >> v;
		g[u].push_back(v);
		g[v].push_back(u);
	}
	int root = 0;
	while(g[root].size() == 1) ++root;
	dfs(root, -1, g, pairs);
	cout << pairs.size() << '\n';
	for(auto p : pairs)
		cout << p.first << ' ' << p.second << '\n';
	return 0;
}