//Date: 2025-01-31 11:01:59
#include<bits/stdc++.h>
#pragma GCC optimize(2)
using namespace std;
#define P emplace_back
#define CLEAR(a,v) memset((a),(v),sizeof((a)))
#define rep(i,a,b) for(int i=(a);i<=(b);i++)
#define per(i,a,b) for(int i=(a);i>=(b);i--)
char buf[1<<20],*p1,*p2;
#define getchar() (p1==p2&&(p2=(p1=buf)+fread(buf,1,1<<20,stdin),p1==p2)?EOF:*p1++)
inline int rd() {
    int s=0,m=0;char ch=getchar();
    while(!isdigit(ch)) {if(ch=='-')m=1;ch=getchar();}
    while( isdigit(ch)) s=(s<<3)+(s<<1)+(ch^48),ch=getchar();
    return m?-s:s;
}
bool MBE;
namespace SOLVER {
#define FORGRAPH for(int i=h[u],v;v=e[i].v;i=e[i].nxt)
int n,fe[100005],f[100005][2],fa[100005],S[100005],T[100005],h[100005],cnt;
int in[100005],out[100005],deg[100005];vector<int> g[100005];queue<int> q;
struct Edge{int v,nxt;} e[200005];inline void add(int u,int v) {e[++cnt]={v,h[u]},h[u]=cnt;}
void dfs(int u,int p) {
    int sum=0,Mn[2]={0x3f3f3f3f,0x3f3f3f3f};
    FORGRAPH if(v!=p) fe[v]=i&1,dfs(v,u),sum+=f[v][0],Mn[fe[v]]=min(Mn[fe[v]],f[v][1]-f[v][0]);
    if(!sum) {f[u][0]=f[u][1]=1;return;}
    f[u][0]=min({sum+Mn[0],sum+Mn[1],sum+Mn[0]+Mn[1]-1}),f[u][1]=min(sum+1,sum+Mn[fe[u]]);
}
void print(int u,int p,int k) {
    int sum=0,Mn[2]={0x3f3f3f3f,0x3f3f3f3f},f0=-1,f1=-1;if(k) fa[u]=fa[p];
    FORGRAPH if(v!=p) sum+=f[v][0],Mn[fe[v]]=min(Mn[fe[v]],f[v][1]-f[v][0]);if(!sum) return;
    if(!k) {
        if(f[u][0]==sum+Mn[0]) f0=1,f1=0;
        if(f[u][0]==sum+Mn[1]) f0=0,f1=1;
        if(f[u][0]==sum+Mn[0]+Mn[1]-1) f0=1,f1=1;
    }
    else {
        if(f[u][1]==sum+1) f0=0,f1=0;
        if(f[u][1]==sum+Mn[fe[u]]) f0=(fe[u]==0),f1=(fe[u]==1);
    }
    FORGRAPH if(v!=p) {
        if(fe[v]==0&&f0&&f[v][1]-f[v][0]==Mn[0]) {print(v,u,1),f0=0;continue;}
        if(fe[v]==1&&f1&&f[v][1]-f[v][0]==Mn[1]) {print(v,u,1),f1=0;continue;}
        print(v,u,0);
    }
}
void MAIN() {
    cin>>n;rep(i,1,n-1) {int u=rd(),v=rd();add(u,v),add(v,u);}
    rep(i,1,n) fa[i]=i;dfs(1,0);print(1,0,0);cout<<f[1][0]-1<<endl;
    for(int i=1,u,v;u=e[i+1].v,v=e[i].v;i+=2) if(fa[u]==fa[v]) out[u]++,in[v]++;else deg[fa[v]]++,g[fa[u]].P(fa[v]);
    rep(i,1,n) {if(!in[i]) S[fa[i]]=i;if(!out[i]) T[fa[i]]=i;if(fa[i]==i&&!deg[i]) q.push(i);}
    int now=0;while(q.size()) {
        int u=q.front();q.pop();if(now) printf("%d %d\n",now,S[u]);now=T[u];
        for(int v:g[u]) if(--deg[v]==0) q.push(v);
    }
}
}
bool MED;
signed main() {
    for(int tt=1;tt;tt--) SOLVER::MAIN();
    return 0;
}