#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
#define fi first
#define se second
#define vi vector<int>
#define vll vector<long long>
#define pii pair<int, int>
#define pll pair<long long, long long>
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define all(x) (x).begin(), (x).end()
#define sz(x) (int)(x).size()
mt19937 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
mt19937_64 rng64(chrono::high_resolution_clock::now().time_since_epoch().count());
inline int rand(int l,int r){return uniform_int_distribution<int>(l, r)(rng);}
inline ll rand(ll l,ll r){return uniform_int_distribution<ll>(l, r)(rng64);}
#ifdef DEBUG
auto&operator<<(auto&o,pair<auto,auto>p){return o<<"("<<p.first<<", "<<p.second<<")";}
auto operator<<(auto&o,auto x)->decltype(x.end(),o){o<<"{";int i=0;for(auto e:x)o<<","+!i++<<e;return o<<"}";}
#define debug(X...)cerr<<"["#X"]: ",[](auto...$){((cerr<<$<<"; "),...)<<endl;}(X)
#else
#define debug(...){}
#endif
struct custom_hash {
static uint64_t splitmix64(uint64_t x) {
x += 0x9e3779b97f4a7c15;
x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
return x ^ (x >> 31);
}
size_t operator()(uint64_t x) const {
static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
return splitmix64(x + FIXED_RANDOM);
}
};
struct pair_hash{
size_t operator()(const pair<int,int>&x)const{
return hash<long long>()(((long long)x.first)^(((long long)x.second)<<32));
}
};
const int MAXN = 2e5 + 7;
vi g[MAXN];
vi gt[MAXN];
vi pos[MAXN];
int done[MAXN];
int vis[MAXN];
vi A[MAXN];
int n, m;
vi f = {};
vector<pii> ans = {};
bool ok = true;
void del(int v, int u, vi& graph){
//cerr << v << ' ' << u << '\n';
//debug(graph);
if(sz(graph) == 1){
graph.clear();
return;
}
if(graph[0] == u){
swap(graph[0], graph[1]);
}
graph.pop_back();
}
void eliminate(int v){
// cerr << "V " << v << '\n';
// debug(g[v]);
// debug(gt[v]);
if(sz(g[v]) > 0 || sz(gt[v]) > 1){
return;
}
if(sz(gt[v]) == 0){
if(pos[v][0] == pos[v][1]){
done[v] = 1;
return;
}
int nowe = pos[v][1];
ans.pb(mp(nowe, pos[v][0]));
pos[v][1] = pos[v][0];
A[nowe].clear();
f.pb(nowe);
done[v] = 1;
return;
}
if(sz(A[pos[v][0]]) > 1){
swap(pos[v][0], pos[v][1]);
}
// debug(pos[v]);
// debug(A[pos[v][1]]);
done[v] = 1;
ans.pb(mp(pos[v][1], pos[v][0]));
int k = A[pos[v][1]][0];
del(k, v, g[k]);
del(v, k, gt[v]);
A[pos[v][1]].pop_back();
pos[v][1] = pos[v][0];
eliminate(k);
}
vi curr;
void dfs(int v){
vis[v] = 1;
curr.pb(v);
if(!vis[g[v][0]]){
dfs(g[v][0]);
}
}
int getBackId(int v){
for(auto ele : pos[v]){
if(v == A[ele].back()){
return ele;
}
}
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(NULL);
cout.tie(NULL);
cin >> n >> m;
//cout << n << ' ' << m << '\n';
for(int i = 1; i <= m; i++){
int a, b;
cin >> a >> b;
// cout << a << ' ' << b << '\n';
if(a == 0){
f.pb(i);
}else if(b == 0){
A[i] = {a};
pos[a].pb(i);
}else{
A[i] = {a, b};
pos[a].pb(i);
pos[b].pb(i);
if(a == b){
done[a] = 1;
}else{
g[a].pb(b);
gt[b].pb(a);
}
}
}
for(int i = 1; i <= n; i++){
eliminate(i);
}
//cerr << "XD\n";
for(int i = 1; i <= n; i++){
if(!done[i] && sz(gt[i]) == 2){
// cerr << i << '\n';
if(sz(f) == 0){
cout << "-1\n";
return 0;
}
for(auto p : pos[i]){
del(A[p][0], i, g[A[p][0]]);
del(i, A[p][0], gt[i]);
}
int xd1 = A[pos[i][0]][0];
int xd2 = A[pos[i][1]][0];
ans.pb(mp(pos[i][0], f.back()));
ans.pb(mp(pos[i][1], f.back()));
A[pos[i][0]].pop_back();
A[pos[i][1]].pop_back();
pos[i][0] = pos[i][1] = f.back();
A[f.back()] = {i, i};
f.pop_back();
done[i] = 1;
eliminate(xd1);
eliminate(xd2);
}
}
// debug(ans);
// for(int i = 1; i <= n; i++){
// cerr << i << '\n';
// debug(g[i]);
// debug(gt[i]);
// debug(pos[i]);
// }
for(int i = 1; i <= n; i++){
if(done[i]){
continue;
}
eliminate(i);
}
// debug(ans);
// debug(f);
//cykle
for(int i = 1; i <= n; i++){
if(sz(g[i]) == 1 && sz(gt[i]) == 1){
// cerr << i << '\n';
// debug(f);
if(sz(f) == 0){
cout << "-1\n";
return 0;
}
if(!vis[i]){
curr = {};
dfs(i);
// debug(curr);
int k = getBackId(i);
// cerr << k << '\n';
//debug(A[k]);
ans.pb(mp(k, f.back()));
A[k].pop_back();
A[f.back()].pb(i);
del(A[k][0], i, g[A[k][0]]);
del(i, A[k][0], gt[i]);
// debug(g[1]);
// debug(gt[1]);
// debug(g[2]);
// debug(gt[2]);
if(pos[i][0] == k){
swap(pos[i][0], pos[i][1]);
}
pos[i][1] = f.back();
// debug(pos[i]);
f.pop_back();
// cerr << "START\n";
eliminate(curr.back());
// debug(ans);
}
}
}
cout << sz(ans) << '\n';
for(auto ele : ans){
cout << ele.fi << ' ' << ele.se << '\n';
}
return 0;
}
