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

using ii = pair<int, int>;
using vi = vector<int>;
using vvi = vector<vi>;
using vii = vector<ii>;

ii dfs(int u, int p, const vvi &E, vii &ans) {
	if (E[u].size() == 1)
		return {u, -1};

	ii ret = {-1, -1};
	for (int v : E[u])
		if (v != p) {
			ii tv = dfs(v, u, E, ans);
			if (ret.first < 0) {
				ret = tv;
				continue;
			}

			if (ret.second < 0 && tv.second < 0)
				ret.second = tv.first;
			else if (ret.second >= 0 && tv.second < 0)
				ans.push_back({ret.second, tv.first}),
				ret.second = -1;
			else if (ret.second < 0 && tv.second >= 0)
				ans.push_back({ret.first, tv.first}),
				ret.first = tv.second;
			else
				ans.push_back({ret.second, tv.first}),
				ret.second = tv.second;
		}
	return ret;
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);

	int n, h;
	cin >> n >> h;
	vvi E(n);
	for (int i = 1, a, b; i < n; ++i)
		cin >> a >> b,
		E[a].push_back(b),
		E[b].push_back(a);

	if (n == 2) {
		cout << "1\n0 1" << endl;
		return 0;
	}

	int u = 0;
	while (E[u].size() < 2) ++u;

	vii ans;
	ans.push_back(dfs(u, -1, E, ans));

	cout << ans.size() << endl;
	for (ii a : ans)
		if (a.second >= 0)
			cout << a.first << ' ' << a.second << '\n';
		else
			cout << a.first << ' ' << u << '\n';

	return 0;
}