#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef pair<int, int> pii;
#define SZ(x) (int) x.size()
#define F first
#define S second
const int N = 5e5 + 10, MOD = 1e9 + 7;
int P[N], S[N], F[N], B[N], H[N], mn[N], M[N], n, m, timer = 0;
vector<pii> adj[N], L; pii E[N];
void preDFS(int v, int p = -1) {
mn[v] = H[v]; M[v] = 1;
for (pii u : adj[v]) {
if (!M[u.F]) {
H[u.F] = H[v] + 1;
preDFS(u.F, v);
mn[v] = min(mn[v], mn[u.F]);
if (mn[u.F] >= H[u.F]) B[u.S] = 1;
} else if (u.F != p) mn[v] = min(mn[v], H[u.F]);
}
}
int Find(int v) {
return !P[v] ? v : P[v] = Find(P[v]);
}
void Union(int u, int v) {
u = Find(u), v = Find(v);
if (u != v) P[u] = v;
return;
}
void DFS(int v, int p = -1) {
S[v] = timer++;
for (pii u : adj[v]) {
if (u.F != p) DFS(u.F, v);
}
}
int main() {
scanf("%d", &n);
if (n == 2) return !printf("-1\n");
m = n - 1;
for (int i = 1; i <= m; i++) {
int u, v; scanf("%d%d", &u, &v);
adj[u].push_back({v, i}); E[i] = {u, v};
adj[v].push_back({u, i});
}
preDFS(1);
for (int i = 1; i <= m; i++) {
if (!B[i]) Union(E[i].F, E[i].S);
}
for (int i = 1; i <= n; i++) adj[i] = {};
for (int i = 1; i <= m; i++) {
if (B[i]) {
int u = Find(E[i].F), v = Find(E[i].S);
adj[u].push_back({v, i});
adj[v].push_back({u, i});
}
}
int rt = 1;
for (int i = 1; i <= n; i++) if (i == Find(i) && SZ(adj[i]) > 1) rt = i;
DFS(rt);
for (int i = 1; i <= n; i++) {
if (i != Find(i)) continue;
if (SZ(adj[i]) == 1) L.push_back({S[i], i});
}
sort(L.begin(), L.end());
if (!SZ(L)) return !printf("0\n");
printf("%d\n", (SZ(L) + 1) / 2);
for (int i = 0; i < (SZ(L) + 1) / 2; i++) {
printf("%d %d\n", L[i].S, L[i + SZ(L) / 2].S);
}
return 0;
}
