#include <algorithm>
#include <iostream>
#include <stack>
#include <vector>

using namespace std;

int main() {
	// Reading the input.
	int n, p;
	cin >> n >> p;
	vector<vector<int>> adj(n);
	for(int i = 0; i < n - 1; i++) {
		int a, b;
		cin >> a >> b;
		adj[a].push_back(b);
		adj[b].push_back(a);
	}

	// Make a list of all the leaves in DFS order.
	stack<int> s;
	s.push(0);
	vector<int> leaves;
	vector<bool> visited(n, false);
	visited[0] = true;

	while(!s.empty()) {
		int cur = s.top();
		s.pop();
		if(adj[cur].size() == 1) leaves.push_back(cur);
		for(int nb : adj[cur]) {
			if(!visited[nb]) {
				visited[nb] = true;
				s.push(nb);
			}
		}
	}

	// Now simply connect leaf i with leaf i + #leaves/2.
	int nleaves = leaves.size();
	cout << (nleaves + 1) / 2 << endl;

	for(int i = 0; i < nleaves / 2; i++) {
		cout << leaves[i] << " " << leaves[i + nleaves / 2] << endl;
	}

	// If we have an odd number of leaves, we have to connect the final one too.
	if(nleaves & 1) { cout << leaves[0] << " " << leaves[nleaves - 1] << endl; }
	return 0;
}