#include<stdio.h>
#include<vector>

#define maxn 100005
#define pb push_back
#define fsize 1200000

using namespace std;

FILE*f=fopen("mesaj4.in","r");
FILE*g=fopen("mesaj4.out","w");

int n,m,i,x,y,k,p,u,nod,nodvcn,size,ch;
int C[maxn],moves1[maxn],moves2[maxn]; bool viz[maxn]; char buff[fsize+5];
vector<int>G[maxn];

inline void Bfs () {
	
	p = u = 1; C[1] = 1; viz[1] = 1;
	
	while ( p <= u ){
		nod = C[p];
		
		for ( 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] ){
			fprintf(g,"%d\n",-1);
			return ;
		}
	}
	
	fprintf(g,"%d\n",k<<1);
	for ( i = k ; i >= 1 ; --i ){
		fprintf(g,"%d %d\n",moves1[i],moves2[i]);
	}
	for ( i = 1 ; i <= k ; ++i ){
		fprintf(g,"%d %d\n",moves2[i],moves1[i]);
	}
}

inline int next () {
	int r = 0;
	
	while ( ch < size && buff[ch+1] >= '0' && buff[ch+1] <= '9' ){
		++ch; r = r * 10 + buff[ch] - '0';
	}
	++ch;
	return r;
}

int main () {
	
	size = fread(buff+1,1,fsize,f); ch = 0;
	n = next(); m = next();
	for ( i = 1 ; i <= m ; ++i ){
		x = next(); y = next();
		G[x].pb(y); G[y].pb(x);
	}
	
	Bfs();
	solutie();
	
	fclose(f);
	fclose(g);
	
	return 0;
}