#include <bits/stdc++.h>
using namespace std;
const int maxn = 200010;
int n, m, c0[maxn], c1[maxn];
bool vis[maxn];
set<pair<int, int>> G[maxn];
vector<array<int, 2>> res;
int main() {
ios::sync_with_stdio(0), cin.tie(0);
cin >> n >> m;
while (m--) {
int u, v;
cin >> u >> v;
G[u].emplace(v, 1), G[v].emplace(u, 0);
c0[u]++, c1[v]++;
}
queue<int> Q;
for (int i = 1; i <= n; i++) {
if (!c0[i] || !c1[i]) Q.push(i);
}
while (!Q.empty()) {
int x = Q.front(); Q.pop();
if (vis[x]) continue;
res.push_back({x, c0[x] ? 1 : 0}), vis[x] = 1;
for (auto &e : G[x]) if (!vis[e.first]) {
(e.second ? c1[e.first] : c0[e.first])--;
G[e.first].erase({x, !e.second});
if (!c0[e.first] || !c1[e.first]) Q.push(e.first);
}
}
if (count(vis + 1, vis + n + 1, 0)) cout << "-1\n", exit(0);
cout << res.size() << "\n";
reverse(res.begin(), res.end());
for (auto &p : res) {
cout << p[0] << " " << p[1] << "\n";
}
return 0;
}
