#include <bits/stdc++.h>
#define pb push_back
#define int long long
using namespace std;
const int mod = 998244353;
const int N = 1e5 + 10;
const int inf = 1e17;
int32_t main(){
int n, m;
cin>>n>>m;
vector<int> in(n+1), out(n+1), adj[n+1];
for(int i = 0; i < m; i++){
int u, v;
cin>>u>>v;
swap(u, v);
out[u]++;
in[v]++;
adj[v].pb(u);
}
queue<int> q;
for(int i = 1; i <= n; i++){
if(!out[i]) q.push(i);
}
vector<int> vis(n+1);
vector<pair<int, int> > ans;
while(!q.empty()){
int x = q.front();
q.pop();
if(vis[x]) continue;
vis[x] = 1;
ans.pb({x, 1});
for(auto itr: adj[x]){
out[itr]--;
if(!out[itr]) q.push(itr);
}
}
if(ans.size() != n){
cout<<-1<<endl;
}
else{
cout<<n<<endl;
reverse(ans.begin(), ans.end());
for(int i = 0; i < n; i++){
cout<<ans[i].first<<" "<<ans[i].second<<endl;
}
}
return 0;
}
