#include <bits/stdc++.h>
using namespace std;
int n, mark;
vector<set<int> > s;
int visited[1009];
struct node {
  int ii, jj;
  int xx, yy;
};
vector<struct node> ss;
int ii, jj;
void print() {
  cout << ss.size() << endl;
  int i;
  for (i = (0); i < (ss.size()); ++i) {
    cout << ss[i].ii + 1 << " " << ss[i].jj + 1 << " " << ss[i].xx + 1 << " "
         << ss[i].yy + 1 << endl;
  }
}
bool check(int n) {
  int i;
  for (i = (0); i < (n); ++i) {
    if (visited[i] == 0) {
      return false;
    }
  }
  return true;
}
void dfs(int index) {
  if (visited[index] == 1) {
    return;
  }
  visited[index] = 1;
  int i;
  set<int>::iterator it;
  for (it = s[index].begin(); it != s[index].end(); it++) {
    dfs((*it));
  }
}
void f(int index, int parent) {
  if (visited[index] != 0) {
    return;
  }
  visited[index] = mark;
  set<int>::iterator it;
  for (it = s[index].begin(); it != s[index].end(); it++) {
    int x = (*it);
    if (x == parent) {
      continue;
    }
    if (visited[x] != 0) {
      if (ii == -1) {
        ii = index;
        jj = x;
      }
    }
    f(x, index);
  }
}
int get(int n) {
  int i;
  for (i = (0); i < (n); ++i) {
    if (visited[i] == 0) {
      return i;
    }
  }
}
void update(int a, int b, int c, int d) {
  s[a].erase(b);
  s[b].erase(a);
  s[c].insert(d);
  s[d].insert(c);
}
int find(int index, int n) {
  int i;
  for (i = (0); i < (n); ++i) {
    if (visited[i] != visited[index]) {
      return i;
    }
  }
}
int main() {
  scanf("%d", &n);
  s.resize(n);
  int i;
  for (i = (0); i < (n - 1); ++i) {
    int x, y;
    scanf("%d", &x);
    scanf("%d", &y);
    x--;
    y--;
    s[x].insert(y);
    s[y].insert(x);
  }
  while (1) {
    memset(visited, 0, sizeof(visited));
    dfs(0);
    if (check(n)) {
      break;
    }
    ii = -1;
    jj = -1;
    memset(visited, 0, sizeof(visited));
    int i;
    mark = 1;
    for (i = (0); i < (n); ++i) {
      if (visited[i] != 0) {
        continue;
      }
      f(i, i);
      mark++;
    }
    struct node ab;
    ab.ii = ii;
    ab.jj = jj;
    ab.xx = ii;
    ab.yy = find(ii, n);
    ss.push_back(ab);
    update(ab.ii, ab.jj, ab.xx, ab.yy);
  }
  print();
  return 0;
}
