// In the name of God
// Hope is last to die
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define pb push_back
// #define int long long
#define S second
#define F first
#define mp make_pair
#define smax(x, y) (x) = max((x), (y))
#define smin(x, y) (x) = min((x), (y))
#define all(x) (x).begin(), (x).end()
#define len(x) ((int)(x).size())
const int maxn = 2e5 + 5;
const int inf = 1e9 + 7;
int n, m, mark[maxn];
vector<int> adj[maxn], topol;
void dfs(int v){
mark[v] = 1;
for(auto u: adj[v]){
if(!mark[u]) dfs(u);
else if(mark[u] == 1){
cout << -1 << '\n';
exit(0);
}
}
topol.pb(v);
mark[v] = 2;
}
void topolo(){
for(int i = 1; i <= n; i++) if(!mark[i]) dfs(i);
reverse(all(topol));
}
int32_t main()
{
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
cin >> n >> m;
for(int i = 0; i < m; i++){
int u, v; cin >> u >> v;
adj[v].pb(u);
}
topolo();
cout << n << '\n';
for(int i = 0; i < n; i++){
cout << topol[i] << ' ' << 1 << '\n';
}
return 0;
}
