#include <bits/stdc++.h>
using namespace std;
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
int n, m;
cin >> n >> m;
vector<vector<int>> adj(n + 1);
for (int i = 0; i < m; i++) {
int u, v;
cin >> u >> v;
adj[u].push_back(v);
}
vector<int> vis(n + 1), topo;
function<void(int)> dfs = [&](int u) {
vis[u] = 1;
for (int v : adj[u]) {
if (!vis[v]) dfs(v);
else if (vis[v] == 1) {
cout << -1;
exit(0);
}
}
topo.push_back(u);
vis[u] = 2;
};
for (int i = 1; i <= n; i++) if (!vis[i]) dfs(i);
cout << n << '\n';
for (int i : topo) cout << i << ' ' << 1 << '\n';
return 0;
}
