#include<bits/stdc++.h>
using namespace std;
const int N=2005,S=41e5;
int u[S],v[S],faa[S],n,m,i,j,a,b,xb,id0,id1,ans,dfn[N],dcnt,dad[N],dep[N],q[N*2],qw;
bool cov[N];
vector<int>e[N];
void dfs(int x,int fa){dfn[++dcnt]=x;dad[x]=fa;dep[x]=dep[fa]+1;for(int y:e[x])if(y!=fa)dfs(y,x);}
inline int lca(int x,int y){return x==y?x:(dep[x]>dep[y]?lca(dad[x],y):lca(x,dad[y]));}
inline void covchain(int b,int a){for(;b!=a;b=dad[b])cov[b]=1;}
inline void work(int x){
	int ii,i;
	for(ii=1;ii<=qw;++ii)if(i=q[ii],u[i]!=x)swap(u[i],v[i]);
	id0=id1=0;if(!cov[x])for(ii=1;ii<=qw;++ii)if(i=q[ii],!faa[i]){if(u[i]!=x)swap(u[i],v[i]);if(u[i]==x)(id0?id1:id0)=i;}
	if(id1){
		if(::i==2){if(v[id0]==v[id1])swap(u[id0],v[id0]);cov[x]=1;return;}
		faa[id0]=faa[id1]=++xb;u[xb]=u[id0]^v[id0]^x;v[xb]=u[id1]^v[id1]^x;
		a=lca(x,u[xb]);b=lca(x,v[xb]);
		if(a==b)covchain(lca(u[xb],v[xb]),a),covchain(x,a);
			else covchain(x,dep[a]>dep[b]?a:b);
		if(u[xb]==v[xb]){faa[xb]=-1;if(u[id0]==u[id1])swap(u[id0],v[id0]);}
			else q[++qw]=xb;
	}else if(id0)++ans;
	for(ii=1;ii<=qw;++ii)if(i=q[ii],!faa[i] && u[i]==x){
		if(v[i]!=dad[x])q[++qw]=faa[i]=++xb,u[xb]=dad[x],v[xb]=v[i];
			else faa[i]=-1;
	}
	int lim=qw;qw=0;
	for(ii=1;ii<=lim;++ii)if(!faa[q[ii]])q[++qw]=q[ii];
}
int main(){
	scanf("%d%d",&n,&m);xb=qw=m;for(i=1;i<=qw;++i)q[i]=i;
	for(i=1;i<n;++i)scanf("%d%d",&a,&b),e[a].push_back(b),e[b].push_back(a);
	for(i=1;i<=m;++i)scanf("%d%d",u+i,v+i);
	dfs(1,0);
	for(i=n;i>1;--i)work(dfn[i]);
	for(i=2;i<=n;++i)ans+=cov[dfn[i]]*2;
	for(i=xb;i;--i)if(faa[i]>0 && (u[i]==v[faa[i]] || v[i]==u[faa[i]]))swap(u[i],v[i]);
	printf("%d\n",ans);
	for(i=1;i<=m;++i)printf("%d %d\n",u[i],v[i]);
	return 0;
}