#include <bits/stdc++.h>
#define int long long
#define pb push_back
#define pii pair<int,int>
#define fr first
#define sc second
using namespace std;
const int N = 2e5+5;
const int M = 5e5+5;
vector<int> adj[N];
vector<int> opp[N];
vector<int> marker;
vector<int> par;
vector<int> keep;
vector<bool> vis;
vector<int> v;
bool dfs(int x) {
marker[x] = 1;
for (auto item : adj[x]) {
if (!marker[item]) {
par[item] = x;
if (dfs(item)) return true;
} else if (marker[item] == 1) return true;
}
marker[x] = 2;
return false;
}
void dfs2(int x) {
vis[x] = 1;
for (auto item : opp[x]) {
if (!vis[item] and adj[item].size() and opp[item].size())
dfs(item);
}
v.push_back(x);
}
signed main() {
int n, m; cin >> n >> m;
marker.assign(n+1, 0);
par.assign(n+1, -1);
vis.assign(n+1, 0);
for (int i = 0; i < m; i++) {
int a, b; cin >> a >> b;
adj[b].pb(a);
opp[a].pb(b);
}
for (int i = 1; i <= n; i++) {
if (!marker[i]) {
if (dfs(i)) {
cout << -1 << endl;
return 0;
}
}
}
vis.assign(n+1, 0);
for (int i = 1; i <= n; i++)
if (!vis[i] and adj[i].size() and opp[i].size())
dfs2(i);
reverse(v.begin(), v.end());
cout << n << endl;
for (auto item : v) {
cout << item << " " << 0 << endl;
}
for (int i = 1; i <= n; i++) {
if (!vis[i]) {
cout << i << " " << (adj[i].size() == 0) << endl;
}
}
}
