#include<fstream>
#include<vector>
#define dmax 100004
using namespace std;
ifstream in("mesaj4.in");
ofstream out("mesaj4.out");

int n,m,ok=1,crt;
bool t[dmax];
vector<int>g[dmax];

struct s
{	int x;
	int y;
}	sol[dmax];

void dfs(int k)
{	vector<int>::iterator it;
	for(it=g[k].begin();it<g[k].end();it++)
		if(!t[*it])
		{	crt++;
			sol[crt].x=k;
			sol[crt].y=*it;
			t[*it]=1;
			dfs(*it);
		}	
}


void solve()
{	int i;
	out<<2*(n-1)<<'\n';
	t[1]=1;
	dfs(1);
	for(i=crt;i>=1;i--)
		out<<sol[i].y<<" "<<sol[i].x<<'\n';
	for(i=1;i<=crt;i++)
		out<<sol[i].x<<" "<<sol[i].y<<'\n';
}


int main()
{	int i,j,a,b;
	in>>n>>m;
	for(i=1;i<=m;i++)
	{	in>>a>>b;
		g[a].push_back(b);
		g[b].push_back(a);
	}
	in.close();
	for(i=1;i<=n;i++)
		if(g[i].empty() )
			ok=0;
	if(!ok)
		out<<"-1";
	else solve();
	out.close();
	return 0;
}