#include <bits/stdc++.h>
// #include <windows.h>
// #include <bits/extc++.h>
// using namespace __gnu_pbds;
using namespace std;
//#pragma GCC optimize(3)
#define DB double
#define LL long long
#define ULL unsigned long long
#define in128 __int128
#define cint const int
#define cLL const LL
#define For(z,e1,e2) for(int z=(e1);z<=(e2);z++)
#define Rof(z,e1,e2) for(int z=(e2);z>=(e1);z--)
#define For_(z,e1,e2) for(int z=(e1);z<(e2);z++)
#define Rof_(z,e1,e2) for(int z=(e2);z>(e1);z--)
#define inint(e) scanf("%d",&e)
#define inll(e) scanf("%lld",&e)
#define inpr(e1,e2) scanf("%d%d",&e1,&e2)
#define in3(e1,e2,e3) scanf("%d%d%d",&e1,&e2,&e3)
#define outint(e) printf("%d\n",e)
#define outint_(e) printf("%d%c",e," \n"[i==n])
#define outint2_(e,e1,e2) printf("%d%c",e," \n"[(e1)==(e2)])
#define outll(e) printf("%lld\n",e)
#define outll_(e) printf("%lld%c",e," \n"[i==n])
#define outll2_(e,e1,e2) printf("%lld%c",e," \n"[(e1)==(e2)])
#define exc(e) if(e) continue
#define stop(e) if(e) break
#define ret(e) if(e) return
#define ll(e) (1ll*(e))
#define pb push_back
#define ft first
#define sc second
#define pii pair<int,int> 
#define pli pair<long long,int> 
#define vct vector 
#define clean(e) while(!e.empty()) e.pop()
#define all(ev) ev.begin(),ev.end()
#define sz(ev) ((int)ev.size())
#define debug(x) printf("%s=%d\n",#x,x)
#define x0 __xx00__
#define y1 __yy11__
#define ffo fflush(stdout)
cLL mod=998244353,G=404;
// cLL mod[2]={1686688681ll,1666888681ll},base[2]={166686661ll,188868881ll};
template <typename Type> void get_min(Type &w1,const Type w2) { if(w2<w1) w1=w2; } template <typename Type> void get_max(Type &w1,const Type w2) { if(w2>w1) w1=w2; }
template <typename Type> Type up_div(Type w1,Type w2) { return (w1/w2+(w1%w2?1:0)); }
template <typename Type> Type gcd(Type X_,Type Y_) { Type R_=X_%Y_; while(R_) { X_=Y_; Y_=R_; R_=X_%Y_; } return Y_; } template <typename Type> Type lcm(Type X_,Type Y_) { return (X_/gcd(X_,Y_)*Y_); }
template <typename Type> Type md(Type w1,const Type w2=mod) { w1%=w2; if(w1<0) w1+=w2; return w1; } template <typename Type> Type md_(Type w1,const Type w2=mod) { w1%=w2; if(w1<=0) w1+=w2; return w1; }
void ex_gcd(LL &X_,LL &Y_,LL A_,LL B_) { if(!B_) { X_=1ll; Y_=0ll; return ; } ex_gcd(Y_,X_,B_,A_%B_); X_=md(X_,B_); Y_=(1ll-X_*A_)/B_; } LL inv(LL A_,LL B_=mod) { LL X_=0ll,Y_=0ll; ex_gcd(X_,Y_,A_,B_); return X_; }
template <typename Type> void add(Type &w1,const Type w2,const Type M_=mod) { w1=md(w1+w2,M_); } void mul(LL &w1,cLL w2,cLL M_=mod) { w1=md(w1*md(w2,M_),M_); } template <typename Type> Type pw(Type X_,Type Y_,Type M_=mod) { Type S_=1; while(Y_) { if(Y_&1) mul(S_,X_,M_); Y_>>=1; mul(X_,X_,M_); } return S_; }
template <typename Type> Type bk(vector <Type> &V_) { auto T_=V_.back(); V_.pop_back(); return T_; } template <typename Type> Type tp(stack <Type> &V_) { auto T_=V_.top(); V_.pop(); return T_; } template <typename Type> Type frt(queue <Type> &V_) { auto T_=V_.front(); V_.pop(); return T_; }
template <typename Type> Type bg(set <Type> &V_) { auto T_=*V_.begin(); V_.erase(V_.begin()); return T_; } template <typename Type> Type bk(set <Type> &V_) { auto T_=*prev(V_.end()); V_.erase(*prev(V_.end())); return T_; }
mt19937 gen(time(NULL)); int rd() { return abs((int)gen()); }
int rnd(int l,int r) { return rd()%(r-l+1)+l; }
void main_init()
{

}
cint N=2.01e5,M=2.01e5,K=20;
int n,m;
vct <int> v[N];
int dep[N],siz[N];
int in[N],out[N],c[N],id;
int up[N],top[N];
void dfs(int p,int fa)
{
    if(fa) v[p].erase(find(all(v[p]),fa));
    dep[p]=dep[fa]+1;
    up[p]=fa;
    siz[p]=1;
    ret(v[p].empty());
    int len=sz(v[p]),k=0;
    For_(i_,0,len)
    {
        int i=v[p][i_];
        dfs(i,p);
        siz[p]+=siz[i];
        if(siz[i]>siz[v[p][k]])
            k=i_;
    }
    swap(v[p][k],v[p][0]);
}
void dfs2(int p,bool opt)
{
    in[p]=out[p]=++id; c[id]=p;
    top[p]=(opt?top[up[p]]:p);
    for(auto i:v[p])
    {
        dfs2(i,i==v[p][0]);
        out[p]=out[i];
    }
}
int lca(int x,int y)
{
    while(top[x]!=top[y])
    {
        if(dep[top[x]]<dep[top[y]]) swap(x,y);
        x=up[top[x]];
    }
    return (dep[x]<dep[y]?x:y);
}
array <int,3> a[M];
int uf[N];
int fin(int w)
{
    if(uf[w]==w) return w;
    return (uf[w]=fin(uf[w]));
}
void mer(int w1,int w2)
{
    w1=fin(w1),w2=fin(w2);
    ret(w1==w2);
    if(w1<w2) swap(w1,w2);
    uf[w2]=w1;
}
vct <int> g[N];
bool check(int p)
{
    return (fin(p)!=p);
}
void fill(int l,int r)
{
    // printf("[%d,%d]\n",l,r);
    // printf("hey\n");
    for(int i=fin(l);i<=r;i=fin(i))
        mer(i,i+1);
    // printf("okay\n");
}
int hav[N];
void main_solve()
{
    inpr(n,m);
    For_(i,1,n)
    {
        int x,y; inpr(x,y);
        v[x].pb(y),v[y].pb(x);
    }
    dfs(1,0);
    id=0; dfs2(1,false);
    For(i,1,m)
    {
        inpr(a[i][0],a[i][1]);
        a[i][2]=lca(a[i][0],a[i][1]);
        // printf("%d\n",a[i][2]);
        g[a[i][2]].pb(i);
    }
    For(i,1,n+1) uf[i]=i;
    int ans=0;
    Rof(num,1,n)
    {
        int p=c[num];
        for(auto i:g[p])
        {
            exc(check(in[a[i][0]])||check(in[a[i][1]]));
            hav[++ans]=p;
            fill(in[p],out[p]);
            break;
        }
    }
    outint(ans);
    For(i,1,ans) outint2_(hav[i],i,ans);
}
int main()
{
    // ios::sync_with_stdio(0); cin.tie(0);
    // freopen("in.txt","r",stdin);
    // freopen("out.txt","w",stdout);
    // srand(time(NULL));
    main_init();
    // int _; inint(_); For(__,1,_) // T>1 ?
        // printf("\n------------\n\n"),
        main_solve();
    return 0;
}
/*

*/