#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
//#include<bits/extc++.h>
//__gnu_pbds
const int N = 2e5+5;
int in[N];
int out[N];
vector<int> side[N];
bool vis[N];
int main(){
ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
int n,m;cin>>n>>m;
for(int i=0;i<m;i++){
int a,b;cin>>a>>b;
in[a]++;
out[b]++;
side[a].push_back(b);
side[b].push_back(a);
}
queue<int> q;
for(int i=1;i<=n;i++){
if(min(in[i],out[i])==0){
q.push(i);
vis[i]=1;
}
}
vector<pair<int,int> > ans;
while(q.size()) {
int cur = q.front();
q.pop();
if(in[cur]) ans.push_back({cur,1});
else ans.push_back({cur,0});
for(auto i : side[cur]){
if(vis[i]) continue;
if(in[cur]){
out[i]--;
}else{
in[i]--;
}
if(min(in[i],out[i])==0){
q.push(i);
vis[i]=1;
}
}
}
for(int i=1;i<=n;i++){
if(!vis[i]){
cout<<-1<<"\n";
return 0;
}
}
cout<<ans.size()<<"\n";
for(int i=(int)ans.size()-1;i>=0;i--){
cout<<ans[i].first<<" "<<ans[i].second<<"\n";
}
return 0;
}
