#include <bits/stdc++.h>
using namespace std;



struct BCC { // find AVs and bridges in an undirected graph
	vector<vector<int>> &edges;
	int V, counter = 0, root; // root and # children of root
	vector<int> n, l;         // nodes,low
	stack<int> s;
	int components = 0;
	BCC(vector<vector<int>> &e) : edges(e), V(e.size()), n(V, -1), l(V, -1) {}
	void visit(int u, int p) { // also pass the parent
		l[u] = n[u] = counter++;
		s.push(u);
		bool skipped_parent = false;
		for(auto &v : edges[u]) {
			if(n[v] == -1) {
				visit(v, u);
				if(l[v] > n[u]) {
					++components;
					while(true) { // biconnected component
						int w = s.top();
						s.pop(); // <= ACT HERE
						if(w == v) break;
					}
				}
				l[u] = min(l[u], l[v]);
			} else if(skipped_parent || v != p) {
				l[u] = min(l[u], n[v]);
			} else { // v==p
				skipped_parent = true;
			}
		}
	}
	int run() {
		for(int u = 0; u < V; ++u)
			if(n[u] == -1) {
				root = u;
				visit(u, -1);
				if(!s.empty()) {
					++components;
					s = stack<int>();
				}
			}
		return components;
	}
};


int main() {
	srand(132423);
	int n, p;
	cin >> n >> p;
	vector<vector<int>> g(n);
	vector<int> d(n,0), leaves;
	for(int i = 0; i < n - 1; ++i) {
		int u, v;
		cin >> u >> v;
		++d[u], ++d[v];
		g[u].push_back(v);
		g[v].push_back(u);
	}
	for(int i = 0; i < n; ++i){
		if(d[i]==1) leaves.push_back(i);
	}
	int ans = (leaves.size() + 1) / 2;
	cout << ans << '\n';
	int loops = 0;
	do {
		random_shuffle(leaves.begin(), leaves.end());
		auto g2 = g;
		for(int i = 0; i < ans; ++i){
			int u = leaves[2*i];
			int v = leaves[(2*i+1)%n];
			g2[u].push_back(v);
			g2[v].push_back(u);
		}
		if(BCC(g2).run() == 1) break;
		++loops;
	} while(true);
	for(int i = 0; i < ans; ++i)
		cout << leaves[2*i] << ' ' << leaves[(2*i+1)%n] << '\n';
	return 0;
}