#include<bits/stdc++.h>
using namespace std;
const int MN = 5e5 + 2;
vector<int> adj[MN], has[MN]; vector<pair<int,int>> ret;
int tt,sz[MN],cmp[MN],freq[MN],cnt,rt=-1;
void dfs (int cur, int p = -1) {
int mx = 0;
for (int i : adj[cur]) if (i != p) {
dfs(i,cur);
mx = max(mx,sz[i]);
sz[cur] += sz[i];
}
if (adj[cur].size()>1) {
mx = max(mx,cnt - sz[cur]);
if (mx * 2 <= cnt) rt = cur;
} else ++sz[cur];
}
void go (int cur, int p = -1, int cc = 0) {
cmp[cur] = cc;
for (int i : adj[cur]) if (i != p) go(i,cur,cc?:i);
if (adj[cur].size() == 1) has[cc].push_back(cur),++freq[cc];
}
int main () {
int n; scanf ("%d",&n);
for (int i = 1; i < n; i++) {
int a,b; scanf ("%d %d",&a,&b);
adj[a].push_back(b); adj[b].push_back(a);
}
for (int i = 1; i <= n; i++) cnt += adj[i].size() == 1;
dfs(1);
assert(~rt);
go(rt);
set<pair<int,int>,greater<pair<int,int>>> st;
for (int i = 1; i <= n; i++) if (freq[i]) st.insert({freq[i],i});
while (!st.empty()) {
if (st.size() == 1) {
auto [f,x] = *st.begin();
ret.emplace_back(has[x].back(),rt);
st.erase(st.begin());
} else {
auto [f,x] = *st.begin();
auto [f2,x2] = *next(st.begin());
ret.emplace_back(has[x].back(),has[x2].back());
has[x].pop_back(); has[x2].pop_back();
st.erase(st.begin()); st.erase(st.begin());
if (--f) st.insert({f,x});
if (--f2) st.insert({f2,x2});
}
}
printf ("%d\n",(int)ret.size());
for (auto [u,v] : ret) printf ("%d %d\n",u,v);
return 0;
}
