#include <bits/stdc++.h>
using namespace std;
#define int long long
const int N=2e5+37;
vector<int> adj[N], radj[N], siz(N), vis(N);
int flag=1;
void dfs(int x, int y)
{
vis[x]=1;
for(auto i: adj[x])
{
if(i==y) continue;
if(vis[i]==1) flag=0;
else if(vis[i]==0)dfs(i, x);
}
vis[x]=2;
}
int32_t main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
int n, m; cin>>n>>m;
for(int i=0; i<m; i++)
{
int x, y; cin>>x>>y;
adj[x].push_back(y);
radj[y].push_back(x);
siz[x]++;
}
queue<int> q;
for(int i=1; i<=n; i++)
{
if(!vis[i]) dfs(i, i);
if(siz[i]==0) q.push(i);
}
if(!flag)
{
cout<<-1; return 0;
}
else
{
vector<array<int, 2>> ans;
while(q.size())
{
int x=q.front(); q.pop();
ans.push_back({x, 1});
for(auto i: radj[x])
{
siz[i]--;
if(siz[i]==0) q.push(i);
}
}
cout<<ans.size()<<"\n";
for(auto i: ans) cout<<i[0]<<" "<<i[1]<<"\n";
}
}
