//Created Time:2020年03月19日 星期四 09时02分09秒
#include <queue>
#include <vector>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#define N 5003

using namespace std;

int n, cnt, m;
int to[N << 3], s[N << 3], t[N << 3], deg[N], fa[N], dep[N], f[N], st[N];
bool vis[N << 3], book[N];

vector<int> e[N], w[N];

void dfs(int ,int);
void get(int ,int);

int main(){
	scanf("%d%d", &n, &m);
	for(int i = 1; i < n; ++i){
		int u, v; scanf("%d%d", &u, &v);
		e[u].push_back(v); e[v].push_back(u);
		++deg[u]; ++deg[v];
	} dfs(1, 0);
	for(int i = 1; i <= m; ++i){
		scanf("%d%d", s + i, t + i);
		to[i] = i; get(s[i], t[i]);
		w[s[i]].push_back(i);
		w[t[i]].push_back(i);
	}
	int ans = 0;
	for(int i = 2; i <= n; ++i)
		if(f[i] > 1) ans += 2;
		else if(f[i] == 1) ++ans;
	printf("%d\n", ans);
	queue<int> Q; int num = m;
	for(int i = 1; i <= n; ++i)
		if(deg[i] == 1) Q.push(i);
	while(!Q.empty()){
		int u = Q.front(), p; Q.pop(); book[u] = true;
		int sz = e[u].size();
		for(int i = 0; i < sz; ++i){
			int v = e[u][i]; --deg[v];
			if(deg[v] == 1) Q.push(v);
			if(!book[v]) p = v;
		}
		sz = w[u].size(); int top = 0;
		for(int i = 0; i < sz; ++i){
			int x = w[u][i]; if(vis[x]) continue;
			st[++top] = x; vis[x] = true;
		}
		if(top & 1){
			int x = st[top--];
			if(s[x] != u) swap(s[x], t[x]);
			if(p != t[x]){
				to[x] = ++num; to[num] = num;
				s[num] = p; t[num] = t[x];
				w[p].push_back(num);
				w[t[x]].push_back(num);
			}
		}
		for(int i = 1; i <= top; i += 2){
			int x = st[i], y = st[i + 1];
			if(s[x] != u) swap(s[x], t[x]);
			if(s[y] != u) swap(s[y], t[y]);
			if(t[x] == t[y]) swap(s[y], t[y]);
			else{
				to[x] = ++num; to[y] = num;
				s[num] = t[x]; t[num] = t[y];
				to[num] = num;
				w[t[x]].push_back(num);
				w[t[y]].push_back(num);
			}
		}
	}
	for(int i = num; i; --i){
		int x = to[i];
		if(s[x] == t[i]) swap(s[i], t[i]);
	}
	for(int i = 1; i <= m; ++i)
		printf("%d %d\n", s[i], t[i]);
	return 0;
}

void dfs(int u, int f){
	fa[u] = f; dep[u] = dep[f] + 1;
	int sz = e[u].size();
	for(int i = 0; i < sz; ++i){
		int v = e[u][i]; if(v == f) continue;
		dfs(v, u);
	} return ;
}

void get(int x, int y){
	if(dep[x] < dep[y]) swap(x, y);
	while(dep[x] > dep[y]) ++f[x], x = fa[x];
	if(x == y) return ;
	while(x != y) ++f[x], ++f[y], x = fa[x], y = fa[y];
	return ;
}