#include<bits/stdc++.h>
using namespace std;
#define rep(i,x,y) for (int i=(x);i<=(y);i++)
#define drep(i,y,x) for (int i=(y);i>=(x);i--)
#define pii pair<int,int>
#define fir first
#define sec second
#define MP make_pair
template<typename T> bool chkmin(T &x,T y){return x>y?x=y,1:0;}
template<typename T> bool chkmax(T &x,T y){return x<y?x=y,1:0;}
void file() {
    #ifdef zqj
    freopen("a.in","r",stdin);
    #endif
}
#define sz 202020

int n;
vector<int>V[sz];

int dfs(int x,int fa) {
    int ww=-1;
    for (auto v:V[x]) if (v!=fa) {
        if (dfs(v,x)) {
            if (ww==-1) ww=v;
            else cout<<ww<<' '<<v<<'\n',ww=-1;
        }
    }
    if (ww!=-1&&fa) return cout<<ww<<' '<<fa<<'\n',0;
    return 1;
}

int main() {
    file();
    ios::sync_with_stdio(false),cin.tie(0);
    cin>>n;
    int x,y;
    rep(i,1,n-1) cin>>x>>y,V[x].push_back(y),V[y].push_back(x);
    cout<<(n-1)/2<<'\n';
    dfs(1,0);
    return 0;
}