#include <bits/stdc++.h>
using namespace std;
#define int long long
#define ll long long
#define ii pair<ll,ll>
#define fi first
#define se second
#define endl '\n'
#define puf push_front
#define pof pop_front
#define pub push_back
#define pob pop_back
#define lb lower_bound
#define ub upper_bound
#define rep(x,s,e) for (auto x=(s)-((s)>(e));x!=(e)-((s)>(e));((s)<(e)?x++:x--))
#define all(x) (x).begin(),(x).end()
#define sz(x) (int) (x).size()
mt19937 rng(chrono::system_clock::now().time_since_epoch().count());
int n,m;
vector<ii> al[200005];
int in[200005];
int out[200005];
bool vis[200005];
vector<int> v;
vector<ii> ans;
signed main(){
cin.tie(0);
cout.tie(0);
cin.sync_with_stdio(false);
cin>>n>>m;
int a,b;
rep(x,0,m){
cin>>a>>b;
al[a].pub({b,0});
al[b].pub({a,1});
in[b]++;
out[a]++;
}
rep(x,1,n+1) if (in[x]==0 || out[x]==0){
vis[x]=true;
v.pub(x);
}
while (!v.empty()){
int u=v.back(); v.pob();
if (in[u]==0) ans.pub({u,1});
else ans.pub({u,0});
for (auto [it,x]:al[u]){
if (!vis[it]){
if (x==0) in[it]--;
else out[it]--;
if (in[it]==0 || out[it]==0){
vis[it]=true;
v.pub(it);
}
}
}
}
reverse(all(ans));
if (sz(ans)==n){
cout<<sz(ans)<<endl;
for (auto [a,b]:ans) cout<<a<<" "<<b<<endl;
}
else{
cout<<"-1"<<endl;
}
}
