#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
const int MAX = 2e6 + 15;
const int INF = 0x3f3f3f3f;
const int MOD = 1e9 + 7;
#define pb push_back
#define sz(x) (int) x.size()
#define fr first
#define sc second
#define mp make_pair
#define all(x) x.begin(), x.end()
#define dbg(x) cerr << #x << ": " << "[ " << x << " ]\n"
int n, m, ingrau[MAX];
vector<int> adj[MAX];
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin >> n >> m;
while(m--){
int u, v; cin >> u >> v;
adj[u].pb(v);
ingrau[v]++;
}
vector<int> q;
for(int i = 1; i <= n; i++)
if(ingrau[i] == 0) q.pb(i);
int cur = 0;
while(cur < sz(q)){
int u = q[cur];
for(auto v : adj[u]){
ingrau[v]--;
if(!ingrau[v])
q.pb(v);
}
cur++;
}
if(sz(q) != n){
cout << -1 << '\n';
}else{
cout << sz(q) << '\n';
for(auto x : q)
cout << x << ' ' << 0 << '\n';
}
return 0;
}
