/*
In the name of Allah
made by: Ahmed_Solyman
*/
#include <bits/stdc++.h>
#include <ext/rope>
using namespace std;
using namespace __gnu_cxx;
#pragma GCC optimize("-Ofast")
#pragma GCC optimize("-O1")
//-------------------------------------------------------------//
typedef long long ll;
typedef unsigned long long ull;
#define fast ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define PI acos(-1)
#define lb lower_bound
#define ub upper_bound
#define endl '\n'
#define all(v) v.begin(),v.end()
#define allr(v) v.rbegin(),v.rend()
#define sum_to(n) (n*(n+1))/2
#define pb push_back
#define pf push_front
#define fil(arr,x) memset(arr,x,sizeof(arr))
const ll mod=1e9+7;
int dx[8]={0,1,0,-1,1,1,-1,-1};
int dy[8]={1,0,-1,0,1,-1,-1,1};
//-------------------------------------------------------------//
ll lcm(ll a,ll b)
{
return (max(a,b)/__gcd(a,b))*min(a,b);
}
void person_bool(bool x)
{
cout<<(x?"YES":"NO")<<endl;
}
vector<int>adj[200005];
vector<int>vis(200005),v;
bool valid=1;
void dfs(int node){
vis[node]=1;
for(int i:adj[node]){
if(!vis[i]){
dfs(i);
}
}
v.push_back(node);
}
int main()
{
//freopen("input.txt","r",stdin);
//freopen("output.txt","w",stdout);
#ifndef ONLINE_JUDGE
//freopen("input.in", "r", stdin);
//freopen("output.out", "w", stdout);
#endif
fast
int n,m;cin>>n>>m;
vector<pair<int,int>>e;
for(int i=0;i<m;i++){
int a,b;cin>>a>>b;
e.push_back({b,a});
adj[b].push_back(a);
}
for(int i=1;i<=n;i++){
if(!vis[i]){
dfs(i);
}
}
reverse(all(v));
map<int,int>pos;
for(int i=0;i<n;i++){
pos[v[i]]=i;
}
for(auto i:e){
valid&=(pos[i.first]<=pos[i.second]);
}
if(!valid){
cout<<"-1"<<endl;
}
else{
cout<<v.size()<<endl;
for(auto i:v)cout<<i<<" "<<1<<"\n";
}
return 0;
}
