#include <iostream>
#include <stack>
#include <string.h>
#include <vector>

using namespace std;

typedef vector <vector<int>> Graph;

vector<int> tsort(Graph G, int start, int end) {
  vector<int> ret;

  int degree[end];
  memset(degree, 0, sizeof(degree));

  for (int from = start; from < end; from++) {
    for (int i = 0; i < G[from].size(); i++) {
      int to = G[from][i];
      degree[to]++;
    }
  }

  stack<int> root;

  for (int i = start; i < end; i++) {
    if (degree[i] == 0) {
      root.push(i);
    }
  }

  while (!root.empty()) {
    int from = root.top();
    root.pop();

    ret.push_back(from);

    for (int i = 0; i < G[from].size(); i++) {
      int to = G[from][i];
      degree[to]--;

      if (degree[to] == 0) {
        root.push(to);
      }
    }
  }

  return ret;
}

int main() {
  int N, M;
  cin >> N >> M;

  Graph G(N);

  int u, v;

  for (int i = 0; i < M; ++i) {
    cin >> u >> v;
    --u;
    --v;

    G[u].push_back(v);
  }

  vector<int> res = tsort(G, 0, N);

  if (res.size() != N) {
    // tsort failed.
    cout << -1 << endl;
  }

  int K = res.size() - 1;
  cout << K << endl;
  for (int i = 0; i < K; ++i) {
    int a = res[i];
    int b = res[i + 1];
    cout << a + 1 << " " << b + 1 << endl;
  }

  return 0;
}