#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 7;
int park[N][2],vis[N],col,sp;
pair < int , int > indx[N][2];
queue < int > top,bottom,consider,space;
signed main(){
int n,m;cin >> n >> m;
for(int i=0;i<m;i++){
cin >> park[i][0] >> park[i][1];
if(vis[park[i][0]]==0){
indx[park[i][0]][0].first = i;
indx[park[i][0]][0].second = 0;
vis[park[i][0]] = 1;
}
else{
indx[park[i][0]][1].first = i;
indx[park[i][0]][1].second = 0;
}
if(vis[park[i][1]]==0){
indx[park[i][1]][0].first = i;
indx[park[i][1]][0].second = 1;
vis[park[i][1]] = 1;
}
else{
indx[park[i][1]][1].first = i;
indx[park[i][1]][1].second = 1;
}
}
int sayac = 0;
vector < pair < int , int > > answer;
fill(vis , vis+N , 0);
for(int i = 0;i<m;i++){
if(park[i][0] == park[i][1]){
if(park[i][0]){
sayac++;
vis[park[i][0]] = 1;
}
else space.push(i);
}
}
for(col = 1;col<=n;col++){
if(indx[col][0].second == 1 and indx[col][1].second == 1)top.push(col);
else if((park[indx[col][0].first][1] == 0 or park[indx[col][0].first][1] == col) and (park[indx[col][1].first][1] == 0 or park[indx[col][1].first][1] == col))bottom.push(col);
}
int bruh = 0;
while(top.size() or bottom.size()){
bruh++;
if(bruh > 4*n){
break;
}
while(bottom.size()){
col = bottom.front();
bottom.pop();
if(vis[col])continue;
vis[col] = 1;
sayac++;
if(indx[col][0].second == 0 and indx[col][1].second == 0){
answer.push_back({indx[col][0].first,indx[col][1].first});
park[indx[col][1].first][1] = col;
park[indx[col][0].first][0] = 0;
space.push(indx[col][0].first);
}
else{
if(indx[col][0].second == 1){
answer.push_back({indx[col][0].first,indx[col][1].first});
park[indx[col][1].first][1] = col;
park[indx[col][0].first][1] = 0;
consider.push(park[indx[col][0].first][0]);
}
else{
answer.push_back({indx[col][1].first,indx[col][0].first});
park[indx[col][0].first][1] = col;
park[indx[col][1].first][1] = 0;
consider.push(park[indx[col][1].first][0]);
}
}
}
while(space.size() and top.size()){
col = top.front();
top.pop();
if(vis[col])continue;
vis[col] = 1;
sayac++;
sp = space.front();
space.pop();
answer.push_back({indx[col][0].first,sp});
answer.push_back({indx[col][1].first,sp});
park[sp][0] = col;
park[sp][1] = col;
park[indx[col][0].first][1] = 0;
park[indx[col][1].first][1] = 0;
consider.push(park[indx[col][0].first][0]);
consider.push(park[indx[col][1].first][0]);
}
while(consider.size()){
col = consider.front();
consider.pop();
if(vis[col])continue;
if(indx[col][0].second == 1 and indx[col][1].second == 1)top.push(col);
else if((park[indx[col][0].first][1] == 0 or park[indx[col][0].first][1] == col) and (park[indx[col][1].first][1] == 0 or park[indx[col][1].first][1] == col))bottom.push(col);
}
}
if(sayac < n)cout << "-1" << "\n";
else{
cout << answer.size() << "\n";
for(auto itr : answer)cout << itr.first+1 << " " << itr.second+1 << "\n";
}
cout << endl;
}
