#include <bits/stdc++.h>
#define IO ios::sync_with_stdio(false);cin.tie(0);
#define debug(x) cerr << #x << " = " << (x) << ' '
#define bug(x) cerr << (x) << ' '
#define endl cerr << '\n'
#define all(v) (v).begin(), (v).end()
#define SZ(v) (ll)(v).size()
#define lowbit(x) (x)&-(x)
#define pb emplace_back
#define F first
#define S second
using namespace std;
using ll = long long;
using pll = pair<ll, ll>;
//#define TEST
const int N = 2e5+5;
int in[N];
vector<int> g[N];
vector<pll> ans;
queue<int> Q;
int main(void)
{ IO
int n, i, m;
cin >> n >> m;
for (i=0; i < m; ++i)
{
int x, y; cin >> y >> x;
g[x].pb(y), ++in[y];
}
int cnt = n;
for (i=1; i <= n; ++i)
{
if (!in[i]) Q.push(i);
}
while (!Q.empty())
{
int x = Q.front(); Q.pop();
--cnt;
ans.pb(x, 0);
for (int v : g[x])
{
--in[v];
if (!in[v]) Q.push(v);
}
}
if (cnt > 0) cout << "-1\n";
else {
cout << SZ(ans) << '\n';
reverse(all(ans));
for (auto p : ans) cout << p.F << ' ' << p.S << '\n';
}
return 0;
}
