#include <bits/stdc++.h>
using namespace std;
void dfs(int u, int p, const vector<vector<int>> &g, vector<int> &leaves) {
	if(g[u].size() == 1) leaves.push_back(u);
	for(auto v : g[u])
		if(v != p) dfs(v, u, g, leaves);
}
int main() {
	int n, p;
	cin >> n >> p;
	vector<vector<int>> g(n);
	vector<int> leaves;
	for(int i = 0; i < n - 1; ++i) {
		int u, v;
		cin >> u >> v;
		g[u].push_back(v);
		g[v].push_back(u);
	}
	for(auto &x : g) sort(x.begin(), x.end());
	dfs(0, -1, g, leaves);
	int ans = (leaves.size() + 1) / 2;
	cout << ans << '\n';
	for(int i = 0; i < ans; ++i)
		cout << leaves[(2 * i + 1) % leaves.size()] << ' ' << leaves[(2 * i + 2) % leaves.size()]
		     << '\n';
	return 0;
}