#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 pos[MAXN];
vi vec[MAXN];
vi f = {};
vector<pii> ans = {};
bool ok = true;
int n, m;
void Move(int p, int q){
if(p == q){
return;
}
ans.pb(mp(p, q));
int x = vec[p].back();
if(pos[x][0] == p){
swap(pos[x][0], pos[x][1]);
}
pos[x][1] = q;
vec[p].pop_back();
vec[q].pb(x);
if(sz(f) && q == f.back()){
f.pop_back();
}
if(sz(vec[p]) == 0){
f.pb(p);
}
}
int eliminate(int v){
if(sz(vec[v]) == 0 || sz(vec[v]) == 2){
return 0;
}
int x = vec[v].back();
int nxt = pos[x][0] + pos[x][1] - v;
//cerr << v << ' ' << nxt << ' ' << x << '\n';
if(vec[nxt].back() == vec[v].back()){
Move(nxt, v);
if(sz(vec[nxt]) == 1){
return eliminate(nxt);
}
return 0;
}else{
return v;
}
}
void eliminate2(int v){
if(sz(vec[v]) == 0 || sz(vec[v]) == 2){
return;
}
int x = vec[v].back();
int nxt = pos[x][0] + pos[x][1] - v;
// cerr << v << ' ' << nxt << ' ' << x << '\n';
// debug(ans);
if(vec[nxt].back() == vec[v].back()){
Move(nxt, v);
if(sz(vec[nxt]) == 1){
eliminate2(nxt);
}
}else{
int our = v;
while(67){
// cerr << our << ' ' << nxt << ' ' << x << '\n';
if(vec[nxt].back() == vec[our].back()){
if(sz(f) == 0){
cout << "-1\n";
exit(0);
}
int xd = f.back();
Move(our, xd);
Move(nxt, xd);
if(sz(vec[nxt])){
eliminate2(nxt);
}
if(sz(vec[our])){
eliminate2(our);
}
return;
}else{
x = vec[nxt].back();
our = nxt;
nxt = pos[x][0] + pos[x][1] - our;
}
}
}
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(NULL);
cout.tie(NULL);
cin >> n >> m;
for(int i = 1; i <= m; i++){
int a, b;
cin >> a >> b;
if(a != 0){
pos[a].pb(i);
vec[i].pb(a);
}else{
f.pb(i);
}
if(b != 0){
pos[b].pb(i);
vec[i].pb(b);
}
}
// debug(ans);
// debug(f);
for(int i = 1; i <= m; i++){
if(sz(vec[i]) == 1){
eliminate(i);
}
}
// cerr << "XD\n";
for(int i = 1; i <= m; i++){
if(sz(vec[i]) == 1){
eliminate2(i);
}
}
// debug(ans);
// debug(f);
for(int i = 1; i <= n; i++){
if(pos[i][0] != pos[i][1] && vec[pos[i][0]].back() == i && vec[pos[i][1]].back() == i){
//jesteś w wierzchołku do którego wchodzi dwóch
// cerr << i << '\n';
// debug(f);
if(sz(f) == 0){
cout << "-1\n";
return 0;
}
int xd1 = pos[i][0];
int xd2 = pos[i][1];
Move(xd1, f.back());
Move(xd2, f.back());
xd1 = eliminate(xd1);
xd2 = eliminate(xd2);
//debug(ans);
if(xd1 && sz(vec[xd1])){
eliminate2(xd1);
}
if(xd2 && sz(vec[xd2])){
eliminate2(xd2);
}
}
}
// cerr << "XD2\n";
//cykle
for(int i = 1; i <= n; i++){
if(pos[i][0] == pos[i][1]){
continue;
}
if(sz(f) == 0){
cout << "-1\n";
return 0;
}
int xd = f.back();
int our = pos[i][0];
if(vec[our].back() != i){
our = pos[i][1];
}
Move(our, xd);
eliminate(xd);
eliminate(our);
}
cout << sz(ans) << '\n';
for(auto ele : ans){
cout << ele.fi << ' ' << ele.se << '\n';
}
return 0;
}
