#include <bits/stdc++.h>
using namespace std;
#define in ({int x=0;int c=getchar(),n=0;for(;!isdigit(c);c=getchar()) n=(c=='-');for(;isdigit(c);c=getchar()) x=x*10+c-'0';n?-x:x;})
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
//mt19937 rng(1012);
int rnd(int l,int r){return l+rng()%(r-l+1);}
#define fasty ios_base::sync_with_stdio(0),cin.tie(0);
#define forinc(a,b,c) for(int a=b,_c=c;a<=_c;++a)
#define fordec(a,b,c) for(int a=b,_c=c;a>=_c;--a)
#define forv(a,b) for(auto&a:b)
#define fi first
#define se second
#define pb push_back
#define ii pair<int,int>
#define mt make_tuple
#define all(a) a.begin(),a.end()
#define reset(f, x) memset(f, x, sizeof(f))
#define bit(x,i) ((x>>(i-1))&1)
#define on(x,i) (x|(1ll<<(i-1)))
#define off(x,i) (x&~(1<<(i-1)))
#define gg exit(0);
const int N=500010;
int n;
vector<int> val;
vector<int> ad[N],adj[N];
vector<ii> ans;
void add(int i,int j){
adj[i].pb(j);
adj[j].pb(i);
ans.pb({i,j});
}
int it,top,scc;
int st[N],ed[N],co[N],w[N],tr[N];
void dfs1(int u){
st[u]=ed[u]=++it; co[u]=1; w[++top]=u;
forv(v,ad[u]) if(v!=tr[u]){
if(co[v]==0){
tr[v]=u;
dfs1(v);
ed[u]=min(ed[u],ed[v]);
} else if(co[v]==1){
ed[u]=min(ed[u],st[v]);
}
}
forv(v,adj[u]) if(v!=tr[u]){
if(co[v]==0){
tr[v]=u;
dfs1(v);
ed[u]=min(ed[u],ed[v]);
} else if(co[v]==1){
ed[u]=min(ed[u],st[v]);
}
}
if(st[u]==ed[u]){
scc++; int v;
do{
v=w[top--];
co[v]=2;
} while(u!=v);
}
}
main(){
#define task "network"
//freopen(task".inp","r",stdin);
//freopen(task".out","w",stdout);
n=in;
forinc(i,2,n){
int u=in,v=in;
ad[u].pb(v);
ad[v].pb(u);
}
forinc(i,1,n) if(ad[i].size()==1) val.push_back(i);
for(;;){
shuffle(all(val),rng);
for(int l=0;l+1<val.size();l+=2)
add(val[l],val[l+1]);
if(val.size()&1)
add(val[0],val.back());
dfs1(1);
if(scc==1)
break;
ans.clear(); it=top=scc=0;
forinc(i,1,n){
co[i]=tr[i]=0;
adj[i].clear();
}
}
//cout<<1;
//gg
cout<<ans.size()<<"\n";
forv(i,ans) cout<<i.fi<<" "<<i.se<<"\n";
}
