#include "bits/stdc++.h"
using namespace std;
#define int long long
const int MAXN = 2e5 + 10;
const int MOD = 1e9 + 7;
mt19937_64 rng((int)std::chrono::steady_clock::now().time_since_epoch().count());
int rnd(int x, int y) {
int u = uniform_int_distribution<int>(x, y)(rng); return u;
}
int bm(int b, int p) {
if(p==0) return 1 % MOD;
int r = bm(b, p >> 1);
if(p&1) return (((r*r) % MOD) * b) % MOD;
return (r*r) % MOD;
}
int inv(int b) {
return bm(b, MOD-2);
}
int fastlog(int x) {
return (x == 0 ? -1 : 64 - __builtin_clzll(x) - 1);
}
void printcase(int i) { cout << "Case #" << i << ": "; }
int n, m;
vector<int> adj[MAXN];
int K[MAXN];
bool vis[MAXN];
stack<int> topsort;
void dfs(int node) {
vis[node] = 1;
for(int x: adj[node]) {
if(!vis[x]) {
dfs(x);
}
}
topsort.push(node);
}
void solve(int tc) {
cin >> n >> m;
vector<pair<int, int> > constraints;
for(int i=1; i<=m; i++) {
int a, b;
cin >> a >> b;
constraints.push_back({a, b});
adj[b].push_back(a);
}
for(int i=1; i<=n; i++) {
if(!vis[i]) {
dfs(i);
}
}
for(int i=1; i<=n; i++) {
K[topsort.top()] = i;
topsort.pop();
}
for(pair<int, int> X: constraints) {
if(K[X.first] < K[X.second]) {
cout << "-1\n"; return;
}
}
int pos[n+1];
for(int i=1; i<=n; i++) pos[K[i]] = i;
cout << n << "\n";
for(int i=1; i<=n; i++) {
cout << pos[i] << " 1\n";
}
}
int32_t main() {
ios::sync_with_stdio(0); cin.tie(0);
int t = 1; //cin >> t;
for(int i=1; i<=t; i++) solve(i);
}
// 勿忘初衷
