#include <bits/stdc++.h>
using namespace std;
# define int long long
# define fir first
# define sec second
# define pb push_back
const int cnst = 2e5+5;
bool mutipletestcase = 0;
//bool debug = false;
int x[cnst];
vector<int> vec[cnst];
void solve() {
int n, m; cin >> n >> m;
for(int i = 1; i<=m; i++) {
int a, b; cin >> a >> b;
vec[b].pb(a);
x[a]++;
}
queue<int> q;
int done = 0;
for(int i = 1; i<=n; i++) {
if(!x[i]) q.push(i), done++;
}
vector<pair<int, int>> ans;
while(!q.empty()) {
int a = q.front(); q.pop();
for(auto v: vec[a]) {
x[v]--;
if(!x[v]) {
q.push(v);
ans.pb({v, 1});
done++;
}
}
if(done == n) break;
}
if(done == n) {
cout << ans.size() << endl;
for(auto v: ans) cout << v.fir << " " << v.sec << endl;
}
else cout << -1 << endl;
}
signed main() {
ios_base::sync_with_stdio(false);
int t = 1;
if(mutipletestcase) cin >> t;
while(t--) solve();
}
