#include<fstream>
#include<vector>

#define maxn 100005
#define pb push_back

using namespace std;

ifstream f("mesaj4.in");
ofstream g("mesaj4.out");

int n,m,i,x,y,k,p,u,nod,nodvcn;
int C[maxn],viz[maxn],moves1[maxn],moves2[maxn];
vector<int>G[maxn];

inline void Bfs () {
	
	p = u = 1; C[1] = 1; viz[1] = 1;
	
	while ( p <= u ){
		nod = C[p];
		
		for ( int i = 0 ; i < G[nod].size() ; ++i ){
			nodvcn = G[nod][i];
			if ( !viz[nodvcn] ){
				viz[nodvcn] = 1;
				C[++u] = nodvcn; 
				++k; moves1[k] = nodvcn; moves2[k] = nod;
			}
		}
		
		++p;
	}
}

inline void solutie () {
	
	for ( i = 1 ; i <= n ; ++i ){
		if ( !viz[i] ){
			g << "-1" << "\n";
			return ;
		}
	}
	
	g << (k<<1) << "\n";
	for ( i = k ; i >= 1 ; --i ){
		g << moves1[i] << " " << moves2[i] << "\n";
	}
	for ( i = 1 ; i <= k ; ++i ){
		g << moves2[i] << " " << moves1[i] << "\n";
	}
}

int main () {
	
	f >> n >> m;
	for ( i = 1 ; i <= m ; ++i ){
		f >> x >> y;
		G[x].pb(y); G[y].pb(x);
	}
	
	Bfs();
	solutie();
	
	f.close();
	g.close();
	
	return 0;
}