#include <cstdio>
#include <vector>

using namespace std;

#define file_in "mesaj4.in"
#define file_out "mesaj4.out"

#define nmax 101000

int N,M;
int a,b,nr,i;
int x[nmax];
int y[nmax];
int viz[nmax];
vector<int> G[nmax];

void dfs(int nod){
	
	if (viz[nod])
		return ;
	
	viz[nod]=1;
	
	vector<int> :: iterator it;
	
	for (it=G[nod].begin();it!=G[nod].end();++it)
		 if (!viz[*it]){
			 	 x[++nr]=nod;
			 y[nr]=*it;
			 dfs(*it);
		 }
}

int main(){
	
	freopen(file_in,"r",stdin);
	freopen(file_out,"w",stdout);
	
	scanf("%d %d", &N,&M);
	for (i=1;i<=M;++i){
		scanf("%d %d", &a, &b);
		
		G[a].push_back(b);
		G[b].push_back(a);
	}
	
	nr=0;
	dfs(1);
	
	if (nr!=N-1){
		printf("-1\n");
		return 0;
	}
	printf("%d\n", 2*N-2);
    for (i=nr;i>=1;--i)	
	   printf("%d %d\n", y[i],x[i]);
	for (i=1;i<=nr;++i)
		 printf("%d %d\n", x[i],y[i]);

	return 0;
	
}