#include <bits/stdc++.h>
using namespace std;
#define int long long
int n, depth[500005], par[500005];
vector<int> g[500005];
void dfs(int v, int p) {
depth[v] = depth[p] + 1;
par[v] = p;
for (const int u : g[v]) {
if (u != p) dfs(u, v);
}
}
int32_t main() {
cin >> n;
for (int i = 1; i < n; i++) {
int u, v; cin >> u >> v;
g[u].push_back(v);
g[v].push_back(u);
}
dfs(1, 1);
int mx = 0, node = 0;
for (int i = 1; i <= n; i++) {
if (depth[i] > mx) mx = depth[i], node = i;
}
fill(depth, depth + n + 1, 0);
dfs(node, node);
mx = 0; int node2 = 0;
for (int i = 1; i <= n; i++) {
if (depth[i] > mx) mx = depth[i], node2 = i;
}
par[node] = -1;
set<int> st;
int cur = node2;
while (cur != -1) {
st.insert(cur);
cur = par[cur];
}
cout << n - mx << '\n';
for (int i = 1; i <= n; i++) {
if (st.find(i) == st.end())
cout << i << ' ' << node << '\n';
}
}
