#include <bits/stdc++.h>
using namespace std;
int main() {
	srand(132423);
	int n, p;
	cin >> n >> p;
	vector<int> d(n,0), leaves;
	for(int i = 0; i < n - 1; ++i) {
		int u, v;
		cin >> u >> v;
		++d[u], ++d[v];
	}
	for(int i = 0; i < n; ++i){
		if(d[i]==1) leaves.push_back(i);
	}
	int ans = (leaves.size() + 1) / 2;
	cout << ans << '\n';
	random_shuffle(leaves.begin(), leaves.end());
	for(int i = 0; i < ans; ++i)
		cout << leaves[2*i] << ' ' << leaves[(2*i+1)%n] << '\n';
	return 0;
}