/*
Author: QAQAutomaton
Lang: C++
Code: E.cpp
Mail: lk@qaq-am.com
Blog: https://www.qaq-am.com/
*/
#include<bits/stdc++.h>
#define debug(...) fprintf(stderr,__VA_ARGS__)
#define DEBUG printf("Passing [%s] in LINE %d\n",__FUNCTION__,__LINE__)
#define Debug debug("Passing [%s] in LINE %d\n",__FUNCTION__,__LINE__)
#define all(x) x.begin(),x.end()
#define x first
#define y second
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
int inf;
const double eps=1e-8;
const double pi=acos(-1.0);
template<class T>int chkmin(T &a,T b){return a>b?a=b,1:0;}
template<class T>int chkmax(T &a,T b){return a<b?a=b,1:0;}
template<class T>T sqr(T a){return a*a;}
template<class T>T mmin(T a,T b){return a<b?a:b;}
template<class T>T mmax(T a,T b){return a>b?a:b;}
template<class T>T aabs(T a){return a<0?-a:a;}
template<class T>int dcmp(T a,T b){return a>b;}
template<int *a>int cmp_a(int x,int y){return a[x]<a[y];}
#define min mmin
#define max mmax
#define abs aabs
struct __INIT__{
	__INIT__(){
		memset(&inf,0x3f,sizeof(inf));
	}
}__INIT___;
namespace io {
	const int SIZE = (1 << 21) + 1;
	char ibuf[SIZE], *iS, *iT, obuf[SIZE], *oS = obuf, *oT = oS + SIZE - 1, c, qu[55]; int f, qr;
	// getchar
	#define gc() (iS == iT ? (iT = (iS = ibuf) + fread (ibuf, 1, SIZE, stdin), (iS == iT ? EOF : *iS ++)) : *iS ++)
	// print the remaining part
	inline void flush () {
		fwrite (obuf, 1, oS - obuf, stdout);
		oS = obuf;
	}
	// putchar
	inline void putc (char x) {
		*oS ++ = x;
		if (oS == oT) flush ();
	}
	template<typename A>
	inline bool read (A &x) {
		for (f = 1, c = gc(); c < '0' || c > '9'; c = gc()) if (c == '-') f = -1;else if(c==EOF)return 0;
		for (x = 0; c <= '9' && c >= '0'; c = gc()) x = x * 10 + (c & 15); x *= f;
		return 1;
	}
	inline bool read (char &x) {
		while((x=gc())==' '||x=='\n' || x=='\r');
		return x!=EOF;
	}
	inline bool read(char *x){
		while((*x=gc())=='\n' || *x==' '||*x=='\r');
		if(*x==EOF)return 0;
		while(!(*x=='\n'||*x==' '||*x=='\r'))*(++x)=gc();
		*x=0;
		return 1;
	}
	template<typename A,typename ...B>
	inline bool read(A &x,B &...y){
		return read(x)&&read(y...);
	}
	template<typename A>
	inline bool write (A x) {
		if (!x) putc ('0'); if (x < 0) putc ('-'), x = -x;
		while (x) qu[++ qr] = x % 10 + '0',  x /= 10;
		while (qr) putc (qu[qr --]);
		return 0;
	}
	inline bool write (char x) {
		putc(x);
		return 0;
	}
	inline bool write(const char *x){
		while(*x){putc(*x);++x;}
		return 0;
	}
	inline bool write(char *x){
		while(*x){putc(*x);++x;}
		return 0;
	}
	template<typename A,typename ...B>
	inline bool write(A x,B ...y){
		return write(x)||write(y...);
	}
	//no need to call flush at the end manually!
	struct Flusher_ {~Flusher_(){flush();}}io_flusher_;
}
using io :: read;
using io :: putc;
using io :: write;
vector<int> to[2005];
map<int,int> t[2005];
int su[2005],sv[2005],fa[2005];
int dep[2005],c[2005],mt;
int rev[4005];
int ls[4005],rs[4005];
int cnt[2005];
int q[2005],*l,*r;
int n,m,u,v;
void dfs(int x,int f){
	fa[x]=f;
	dep[x]=dep[f]+1;
	for(auto i:to[x])if(i!=f){
		++cnt[x];
		dfs(i,x);
	}
}
void add(int u,int v,int w){
	if(u==v)return;
	if(t[u].count(v)){
		int x=t[u][v];
		t[u].erase(v);
		t[v].erase(u);
		if(w<0)rev[-w]=0;
		else rev[w]=1;
		if(x<0)rev[-x]=1;
		else rev[x]=0;
	}
	else{
		t[u][v]=w;t[v][u]=-w;
	}
}
void link(int u,pii x,pii y){
	x.y=-x.y;
	++mt;
	ls[mt]=abs(x.y);
	rs[mt]=abs(y.y);

	if(x.y<0)rev[-x.y]=1;
	else rev[x.y]=0;
	if(y.y<0)rev[-y.y]=1;
	else rev[y.y]=0;
	add(x.x,y.x,mt);
}
void work(){
	l=q+1,r=q;
	for(int i=1;i<=n;++i)if(!cnt[i])*(++r)=i;
	while(l<=r){
		u=*l;
		if(u==1)break;
		while(t[u].size()){
			pii x=*t[u].begin();
			t[u].erase(t[u].begin());
			t[x.x].erase(u);
			if(t[u].empty()){
				add(fa[u],x.x,x.y);	
			}
			else{
				pii y=*t[u].begin();
				t[u].erase(t[u].begin());
				t[y.x].erase(u);
				link(u,x,y);
			}
		}
		if(!--cnt[fa[u]])*(++r)=fa[u];
		++l;
	}
}
signed main(){
#ifdef QAQAutoMaton 
	freopen("E.in","r",stdin);
	freopen("E.out","w",stdout);
#endif
	read(n,m);
	for(int i=1;i<n;++i){
		read(u,v);
		to[u].push_back(v);
		to[v].push_back(u);
	}
	for(int i=1;i<=m;++i){
		read(su[i],sv[i]);
		add(su[i],sv[i],i);
	}
	mt=m;
	dfs(1,0);
	int ans=0;
	for(int i=1;i<=m;++i){
		u=su[i];v=sv[i];
		while(u!=v){
			if(dep[u]<dep[v])swap(u,v);
			++c[u];u=fa[u];
		}
	}
	for(int i=2;i<=n;++i){
		ans+=min(c[i],2);
	}

	write(ans,'\n');
	work();
	for(int i=mt;i>m;--i){
		rev[ls[i]]^=rev[i];
		rev[rs[i]]^=rev[i];
	}
	for(int i=1;i<=m;++i)if(rev[i])write(su[i],' ',sv[i],'\n');
	else write(sv[i],' ',su[i],'\n');
	return 0;
}



Measured memory_bytes: 2816