#include <bits/stdc++.h>
inline long long in() {
  long long x = 0;
  char c = getchar();
  bool neg = false;
  while (c != '-' && ('0' > c || c > '9')) c = getchar();
  if (c == '-') neg = true, c = getchar();
  while ('0' <= c && c <= '9') x = 10 * x + c - '0', c = getchar();
  if (neg) x = -x;
  return x;
}
inline void out(long long x) {
  if (x < 0) putchar('-'), x = -x;
  if (x > 9) out(x / 10);
  putchar(x % 10 + '0');
}
using namespace std;
const int N = 1010;
int n, it;
int v, e;
int id[N], ed[N], ve[N], p[N];
bool dd[N], fre[N];
vector<pair<int, int> > g;
vector<pair<int, int> > ad[N];
stack<int> st;
stack<pair<int, pair<int, int> > > sc;
struct oo {
  int a, b, c, d;
};
vector<oo> ans;
int root(int x) { return id[x] < 0 ? x : id[x] = root(id[x]); }
void unite(int x, int y) {
  if ((x = root(x)) == (y = root(y))) return;
  id[x] += id[y];
  id[y] = x;
}
void dfs(int u) {
  p[u] = it;
  if (!dd[u]) v++, dd[u] = true;
  for (auto& v : ad[u])
    if (!fre[v.second]) {
      fre[v.second] = true;
      dfs(v.first);
      g.push_back({u, v.first});
    }
}
int main() {
  n = in();
  for (int i = 2, _c = n; i <= _c; ++i) {
    int u = in(), v = in();
    ad[u].push_back({v, i});
    ad[v].push_back({u, i});
  }
  memset(id, -1, sizeof(id));
  for (int i = 1, _c = n; i <= _c; ++i)
    if (!dd[i]) {
      it++;
      g.clear(), v = 0, dfs(i), e = g.size();
      ve[it] = v, ed[it] = e;
      if (v == e + 1)
        st.push(i);
      else
        for (auto& x : g) {
          int u = x.first, v = x.second;
          if (root(u) == root(v))
            sc.push({i, {u, v}});
          else
            unite(u, v);
        }
    }
  if (st.size() == 1 && sc.empty()) return out(0), 0;
  while (!st.empty() && !sc.empty()) {
    pair<int, pair<int, int> > x = sc.top();
    sc.pop();
    ans.push_back({x.second.second, x.second.first, x.first, st.top()});
    st.pop();
    ed[p[x.second.second]]--;
    if (ed[p[x.second.second]] == ve[p[x.second.second]] - 1)
      st.push(x.second.second);
  }
  out(ans.size()), putchar('\n');
  for (auto& x : ans)
    out(x.a), putchar(' '), out(x.b), putchar(' '), out(x.c), putchar(' '),
        out(x.d), putchar('\n');
}
