#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#define pb push_back
#define int long long
#define repp(i,x,n) for(int i=x;i<=n;i++)
#define rep(i,x,n) for(int i=x;i>=n;i--)
#define r0 return 0
#define fi first
#define se second
#define liow ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
#define jelek cout<<"jelek"<<endl
#define pii pair<int,int>
#define all(v) v.begin(),v.end()
#define tp tuple<int,int,int>
#define fl fflush(stdout)
#define ld long double
#define p5 pair<int,pair<pair<int,int>,pair<int,int>>>
#pragma GCC optimize ("O2")
#pragma GCC optimize ("unroll-loops")
#define pil pair<int,int>
#define p3 pair<int,pii>
using namespace std;
using namespace __gnu_pbds;
typedef tree<int,null_type,less<int>,rb_tree_tag,tree_order_statistics_node_update> ordered_set;
typedef tree<pair<int,int>, null_type, less<pair<int,int>>, rb_tree_tag, tree_order_statistics_node_update> ordered_multiset;
//const int mod=1e9+7;
const int SQMAX=635,INF=1e18;
const int mod=998244353;
//const int MOD=1e6+3;
mt19937_64 rng((unsigned int) chrono::steady_clock::now().time_since_epoch().count());
pii dr[8]={{-1,0},{-1,1},{0,1},{1,1},{1,0},{1,-1},{0,-1},{-1,-1}};
int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1};
int dxx[4]={1,1,-1,-1};
int dyy[4]={1,-1,-1,1};
int mul(int x,int y){return (x%mod*y%mod)%mod;}
int expo(int b,int e){
if(e==0) return 1;
int tmp=expo(b,e/2);
if(e%2) return mul(tmp,mul(tmp,b));
else return mul(tmp,tmp);
}
const int maxn=200005;
const int N=1e5;
vector<int>adj[maxn];
void solve(){
int n,m;cin>>n>>m;
vector<bool>mau(n+2);
vector<int>deg(n+2);
repp(i,1,m){
int u,v;cin>>u>>v;
adj[v].pb(u);
deg[u]++;
mau[u]=1;
}
queue<int>q;
repp(i,1,n){
if(!mau[i]){
q.push(i);
}
}
int brp=0;
repp(i,1,n) if(mau[i]) brp++;
vector<pii>ans;
while(!q.empty()){
int x=q.front();q.pop();
if(!mau[x]){
ans.pb({x,1});
}
for(auto v:adj[x]){
if(deg[v]==0) continue;
deg[v]--;
if(deg[v]==0){
brp--;
ans.pb({v,0});
q.push(v);
}
}
}
if(brp!=0){
cout<<-1<<endl;
return;
}
cout<<ans.size()<<endl;
for(auto[x,v]:ans) cout<<x<<" "<<v<<endl;
}
signed main(){
liow;
int t=1;
// cin>>t;
while(t--){
solve();
}
}
