#include <bits/stdc++.h>
using namespace std;
int main() {
	int n, p;
	cin >> n >> p;
	vector<int> g(n);
	vector<int> leaves;
	for(int i = 0; i < n - 1; ++i) {
		int u, v;
		cin >> u >> v;
		++g[u], ++g[v];
	}
	for(int i = 0; i < n; ++i)
		if(g[i] == 1) leaves.push_back(i);
	srand(123);
	std::random_shuffle(leaves.begin(), leaves.end());
	int ans = (leaves.size() + 1) / 2;
	cout << ans << '\n';
	for(int i = 0; i < ans; ++i)
		cout << leaves[2 * i] << ' ' << leaves[(2 * i + 1) % leaves.size()] << '\n';
	return 0;
}