//Author:xht37
#include <bits/stdc++.h>

#define ui unsigned int
#define ll long long
#define ul unsigned ll
#define ld long double

#define pi pair <int, int>
#define fi first
#define se second
#define mp make_pair

#define ls (p << 1)
#define rs (ls | 1)
#define md ((t[p].l + t[p].r) >> 1)

#define vi vector <int>
#define pb push_back
#define pq priority_queue

#define dbg(x) cerr << #x" = " << x << endl
#define debug(...) fprintf(stderr, __VA_ARGS__)

#define fl(x) freopen(x".in", "r", stdin), freopen(x".out", "w", stdout)

using namespace std;

namespace io {
	const int SI = 1 << 21 | 1;
	char IB[SI], *IS, *IT, OB[SI], *OS = OB, *OT = OS + SI - 1, c, ch[100];
	int f, t;
	#define gc() (IS == IT ? (IT = (IS = IB) + fread(IB, 1, SI, stdin), IS == IT ? EOF : *IS++) : *IS++)
	inline void flush() {
		fwrite(OB, 1, OS - OB, stdout), OS = OB;
	}
	inline void pc(char x) {
		*OS++ = x;
		if (OS == OT) flush();
	}

	template <class I>
	inline void rd(I &x) {
		for (f = 1, c = gc(); c < '0' || c > '9'; c = gc()) if (c == '-') f = -1;
		for (x = 0; c >= '0' && c <= '9'; x = (x << 3) + (x << 1) + (c & 15), c = gc());
		x *= f;
	}
	template <class I>
	inline void rd(I &x, I &y) {
		rd(x), rd(y);
	}
	template <class I>
	inline void rd(I &x, I &y, I &z) {
		rd(x), rd(y), rd(z);
	}
	template <class I>
	inline void rda(I *a, int n) {
		for (int i = 1; i <= n; i++) rd(a[i]);
	}
	inline void rdc(char &c) {
		for (c = gc(); c < 33 || c > 126; c = gc());
	}
	inline void rds(char *s, int &n) {
		for (c = gc(); c < 33 || c > 126; c = gc());
		for (n = 0; c >= 33 && c <= 126; s[++n] = c, c = gc());
		s[n+1] = '\0';
	}
	inline void rds(string &s) {
		for (c = gc(); c < 33 || c > 126; c = gc());
		for (s.clear(); c >= 33 && c <= 126; s.pb(c), c = gc());
	}

	template <class I>
	inline void print(I x, char k = '\n') {
		if (!x) pc('0');
		if (x < 0) pc('-'), x = -x;
		while (x) ch[++t] = x % 10 + '0', x /= 10;
		while (t) pc(ch[t--]);
		pc(k);
	}
	template <class I>
	inline void print(I x, I y) {
		print(x, ' '), print(y);
	}
	template <class I>
	inline void print(I x, I y, I z) {
		print(x, ' '), print(y, ' '), print(z);
	}
	template <class I>
	inline void printa(I *a, int n) {
		for (int i = 1; i <= n; i++) print(a[i], " \n"[i==n]);
	}
	inline void printc(char c) {
		pc(c);
	}
	inline void prints(char *s, int n) {
		for (int i = 1; i <= n; i++) pc(s[i]);
		pc('\n');
	}
	inline void prints(string s) {
		int n = s.length();
		while (t < n) pc(s[t++]);
		pc('\n'), t = 0;
	}
	struct Flush {
		~Flush() {
			flush();
		}
	} flusher;
}
using io::rd;
using io::rda;
using io::rdc;
using io::rds;
using io::print;
using io::printa;
using io::printc;
using io::prints;

const int N = 4e3 + 5;
int n, m, de[N], fa[N], ans;
vi e[N];
int u[N], v[N], fu[N], fv[N], f[N], g[N], tot;
bool w[N], o[N], r[N];

void dfs(int x) {
	for (int y : e[x])
		if (y != fa[x]) fa[y] = x, de[y] = de[x] + 1, dfs(y);
}

void mark(int a, int b, int c, int d) {
	static int t[N];
	memset(t, 0, sizeof(t));
	while (a != b) {
		if (de[a] < de[b]) swap(a, b);
		++t[a], a = fa[a];
	}
	while (c != d) {
		if (de[c] < de[d]) swap(c, d);
		++t[c], c = fa[c];
	}
	for (int i = 1; i <= n; i++) o[i] |= t[i] >= 2;
}

void work(int x) {
	for (int y : e[x])
		if (y != fa[x]) work(y);
	if (x == 1) return;
	int t = 0;
	for (int i = 1; i <= tot; i++)
		if (!w[i]) t += (fu[i] == x) ^ (fv[i] == x);
	ans += min(max(t, o[x] * 2), 2);
	if (t <= 1 || o[x]) {
		for (int i = 1; i <= tot; i++)
			if (!w[i]) {
				if (fu[i] == x) fu[i] = fa[x];
				if (fv[i] == x) fv[i] = fa[x];
			}
		return;
	}
	vi p;
	for (int i = 1; i <= tot; i++)
		if (!w[i] && ((fu[i] == x) ^ (fv[i] == x))) p.pb(i);
	int a = p[0], b = p[1];
	mark(fu[a], fv[a], fu[b], fv[b]);
	w[a] = w[b] = 1, f[++tot] = a, g[tot] = b;
	if (fu[a] == x) u[tot] = v[a], fu[tot] = fv[a];
	else u[tot] = u[a], fu[tot] = fu[a];
	if (fu[b] == x) v[tot] = v[b], fv[tot] = fv[b];
	else v[tot] = u[b], fv[tot] = fu[b];
	for (int i = 1; i <= tot; i++)
		if (!w[i]) {
			if (fu[i] == x) fu[i] = fa[x];
			if (fv[i] == x) fv[i] = fa[x];
		}
}

int main() {
	rd(n, m), tot = m;
	for (int i = 1, x, y; i < n; i++)
		rd(x, y), e[x].pb(y), e[y].pb(x);
	de[1] = 1, dfs(1);
	for (int i = 1; i <= m; i++)
		rd(u[i], v[i]), fu[i] = u[i], fv[i] = v[i];
	work(1), print(ans);
	for (int i = tot; i; i--) {
		if (!w[i]) r[i] = 1;
		if (i > m) {
			int a = f[i], b = g[i];
			r[a] = r[i] ^ (u[i] != u[a]);
			r[b] = r[i] ^ (v[i] != v[b]);
		}
	}
	for (int i = 1; i <= m; i++)
		if (r[i]) print(u[i], v[i]);
		else print(v[i], u[i]);
	return 0;
}

Measured memory_bytes: 3996