#include <bits/stdc++.h>
//#include "includeall.h"
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>
#define endl '\n'
#define f first
#define s second
#define pb push_back
#define mp make_pair
#define lb lower_bound
#define ub upper_bound
#define input(x) scanf("%lld", &x);
#define input2(x, y) scanf("%lld%lld", &x, &y);
#define input3(x, y, z) scanf("%lld%lld%lld", &x, &y, &z);
#define input4(x, y, z, a) scanf("%lld%lld%lld%lld", &x, &y, &z, &a);
#define print(x, y) printf("%lld%c", x, y);
#define show(x) cerr << #x << " is " << x << endl;
#define show2(x,y) cerr << #x << " is " << x << " " << #y << " is " << y << endl;
#define show3(x,y,z) cerr << #x << " is " << x << " " << #y << " is " << y << " " << #z << " is " << z << endl;
#define all(x) x.begin(), x.end()
#define discretize(x) sort(x.begin(), x.end()); x.erase(unique(x.begin(), x.end()), x.end());
#define FOR(i, x, n) for (ll i =x; i<=n; ++i)
#define RFOR(i, x, n) for (ll i =x; i>=n; --i)
using namespace std;
mt19937_64 rnd(chrono::steady_clock::now().time_since_epoch().count());
//using namespace __gnu_pbds;
//#define ordered_set tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>
//#define ordered_multiset tree<int, null_type, less_equal<int>, rb_tree_tag, tree_order_statistics_node_update>
typedef long long ll;
typedef long double ld;
typedef pair<ld, ll> pd;
typedef pair<string, ll> psl;
typedef pair<ll, ll> pi;
typedef pair<ll, pi> pii;
typedef pair<pi, pi> piii;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
vector<ll> adj[200005], radj[200005];
vector<ll> order, status;
bool fail = false;
void topo(ll node)
{
if (node==0) return;
if (status[node]==2) return;
if (status[node]==1) {fail = true; return;}
status[node]=1;
for (ll x: adj[node])
{
topo(x);
if (fail) break;
}
status[node]=2;
order.pb(node);
}
int main()
{
ll n, m;
cin >> n >> m;
for (ll i=0; i<m; ++i)
{
ll u, v;
cin >> u >> v;
adj[v].pb(u);
radj[u].pb(v);
}
status.assign(n + 1, 0);
for (ll i=1; i<=n; ++i) if (status[i]==0) topo(i);
reverse(all(order));
//for (ll u: order) show(u);
if (fail) cout << -1 << endl;
else
{
vector<ll> ans;
for (ll i=0; i<n; ++i) if (radj[order[i]].size()) ans.pb(order[i]);
cout << ans.size() << endl;
for (ll u: ans) cout << u << " 1" << endl;
}
return 0;
}
