#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("sse4,abm,avx,popcnt")
#include<bits/stdc++.h>
#define int long long
#define quick ios::sync_with_stdio(0);cin.tie(0);
#define rep(x,a,b) for(int x=a;x<=b;x++)
#define repd(x,a,b) for(int x=a;x>=b;x--)
#define F first
#define S second
#define eb emplace_back
#define mp make_pair
#define mt make_tuple
#define all(x) x.begin(),x.end()
#define sz(x) (int)(x.size())
#define lowbit(x) (x&-x)
using namespace std;
typedef pair<int,int> pii;
void debug(){
cout<<"\n";
}
template<class T,class ...U>
void debug(T a,U ...b){
cout<<a<<" ",debug(b...);
}
const int N=2e5+7;
const int INF=1e18L;
vector<int> v[N];
int out[N];
bool lgt[N];
signed main(){
quick
int n,m;
cin>>n>>m;
rep(i,1,m){
int a,b;
cin>>a>>b;
v[b].eb(a);
lgt[a]=true;
out[a]++;
}
queue<int> q;
vector<pii> ans;
rep(i,1,n){
if(!out[i]) q.push(i);
}
while(sz(q)){
int x=q.front();
q.pop();
ans.eb(x,lgt[x]);
for(int j:v[x]){
out[j]--;
if(!out[j]) q.push(j);
}
}
if(sz(ans)!=n) cout<<"-1\n";
else{
cout<<sz(ans)<<"\n";
for(pii p2:ans) cout<<p2.F<<" "<<p2.S<<"\n";
}
}
