#include <bits/stdc++.h>
using namespace std;
#define v vector
#define int long long
#define all(a) (a).begin(), (a).end()
#define pb push_back
#define mk make_pair
#define pii pair<int, int>
#define ff first
#define ss second
#define inp_v(vec) for (auto &i : vec) cin >> i;
#define prt_v(vec) for (auto i : vec) cout << i << endl;
#define MS(x) memset(x, 0, sizeof(x))
#define gcd(a,b) __gcd(a, b);
#define lcm(a,b) (a*(b/gcd(a,b)))
#define lb lower_bound
#define ub upper_bound
bool chmin(int &a, int b){return b<a?a=b, true:false;}
bool chmax(int &a, int b){return b>a?a=b, true:false;}
const int maxn=5e5+5;
vector<int>adj[maxn];
bool vis[maxn];
vector<int>val(maxn);
signed main(){
ios_base::sync_with_stdio(false); cin.tie(0);
int N, M;cin>>N>>M;
vector<pair<int, int>>A(M+1);
// vector<int>val(N+1);
for(int i=0;i<M;i++){
cin>>A[i].first>>A[i].second;
adj[A[i].first].push_back(A[i].second);
val[A[i].second]++;
}
queue<int>q;
vector<pair<int, int>>ans;
for(int i=1;i<=N;i++){
if(!val[i]){
q.push(i);
// ans.push_back({i, 0});
vis[i]=true;
}
}
while(!q.empty()){
int p=q.front();
q.pop();
for(auto i:adj[p]){
val[i]--;
if(!val[i]){
q.push(i);
ans.push_back({i, 0});
}
}
}
bool stat=true;
for(int i=1;i<=N;i++){
// cout<<i<<" "<<val[i]<<endl;
if(val[i])stat=false;
}
// stat=true;
if(!stat)cout<<-1<<endl;
else{
cout<<ans.size()<<endl;
for(auto i:ans){
cout<<i.first<<" "<<i.second<<endl;
}
}
return 0;
}
