// #pragma GCC target ("avx,avx2,fma")
// #pragma GCC optimize ("Ofast,inline") // O1 - O2 - O3 - Os - Ofast
// #pragma GCC optimize ("unroll-loops")
#include <bits/stdc++.h>
using namespace std;
#define rep(i, a, b) for (ll i = (a); i < (b); ++i)
#define per(i, a, b) for (ll i = (b) - 1; i >= (a); --i)
#define trav(a, x) for (auto &a : x)
#define all(x) x.begin(), x.end()
#define sz(x) (ll)x.size()
#define pb push_back
#define debug(x) cout<<#x<<" = "<<x<<endl
#define umap unordered_map
#define uset unordered_set
typedef pair<int, int> ii;
typedef pair<int, ii> iii;
typedef vector<int> vi;
typedef vector<ii> vii;
typedef vector<vi> vvi;
typedef long long ll;
typedef pair<ll,ll> pll;
typedef pair<ll,pll> ppll;
typedef vector<ll> vll;
typedef vector<pll> vpll;
typedef vector<vll> vvll;
const ll INF = 1'000'000'007;
ll n,m;
vector<umap<ll,ll>> graph;
vector<ll> value;
vector<pll> order;
int main() {
cin.tie(0)->sync_with_stdio(0);
cin.exceptions(cin.failbit);
cin>>n>>m;
graph.resize(n);
rep(i,0,m) {
ll u,v;
cin>>u>>v;
--u; --v;
graph[u].emplace(v,1);
graph[v].emplace(u,0);
}
value.assign(n,0);
queue<ll> q;
vector<bool> vis;
vis.assign(n,0);
rep(u,0,n) {
trav(edge,graph[u]) value[u]+=edge.second;
if(value[u]%sz(graph[u])==0) {
q.push(u);
vis[u]=1;
}
}
while(!q.empty()) {
ll u=q.front();
q.pop();
if(sz(graph[u])) order.emplace_back(u,value[u]/sz(graph[u]));
else order.emplace_back(u,0);
trav(edge,graph[u]) {
ll v,w;
tie(v,w)=edge;
graph[v].erase({u});
value[v]-=1-w;
if(!vis[v]&&(sz(graph[v])==0||value[v]%sz(graph[v])==0)) {
vis[v]=1;
q.push(v);
}
}
}
if(sz(order)!=n) cout<<-1<<endl;
else {
cout<<n<<endl;
reverse(all(order));
trav(item,order) cout<<item.first+1<<' '<<item.second<<'\n';
}
return 0;
}
