#include <bits/stdc++.h>
using namespace std;
vector<int> ady[1005];
int dfsNum[1005], V, low[1005], num, seen[1005], num_component,
    visto[1005][1005], root[1005];
vector<pair<int, int> > back_edge[1005];
void dfs(int x, int parent) {
  dfsNum[x] = low[x] = num++;
  seen[x] = 1;
  for (int i = 0; i < ady[x].size(); ++i) {
    int y = ady[x][i];
    if (!seen[y]) {
      dfs(y, x);
      if (low[x] > low[y]) low[x] = low[y];
    } else if (y != parent) {
      if (!visto[x][y] && !visto[y][x]) {
        visto[x][y] = visto[y][x] = 1;
        back_edge[num_component].push_back(make_pair(x, y));
      }
      low[x] = (low[x] > dfsNum[y]) ? dfsNum[y] : low[x];
    }
  }
}
int uf[1005];
void MakeSet() {
  for (int i = 1; i <= V; ++i) uf[i] = i;
}
int Find(int x) { return (uf[x] == x) ? x : uf[x] = Find(uf[x]); }
void Union(int x, int y) { uf[Find(x)] = Find(y); }
int main() {
  int t, u, v, i, j;
  scanf("%d", &V);
  MakeSet();
  for (i = 0; i < V - 1; ++i) {
    scanf("%d %d", &u, &v);
    ady[u].push_back(v);
    ady[v].push_back(u);
    Union(u, v);
  }
  memset(seen, 0, sizeof(seen));
  memset(visto, 0, sizeof(visto));
  num_component = 0;
  for (i = 1; i <= V; ++i) {
    if (!seen[i]) {
      num = 1;
      dfs(i, -1);
      root[num_component++] = i;
    }
  }
  printf("%d\n", num_component - 1);
  for (i = 0; i < num_component; ++i) {
    for (j = 0; j < num_component; ++j) {
      if (i == j) continue;
      if (Find(root[i]) != Find(root[j])) {
        if (back_edge[i].size() > 0) {
          pair<int, int> pii = back_edge[i].back();
          back_edge[i].pop_back();
          printf("%d %d %d %d\n", pii.first, pii.second, pii.first, root[j]);
          Union(root[i], root[j]);
        } else if (back_edge[j].size() > 0) {
          pair<int, int> pii = back_edge[j].back();
          back_edge[j].pop_back();
          printf("%d %d %d %d\n", pii.first, pii.second, pii.first, root[i]);
          Union(root[i], root[j]);
        }
      }
    }
  }
  return 0;
}
