#include <fstream>
#include <vector>
#include <queue>

using namespace std;

const char InFile[]="mesaj4.in";
const char OutFile[]="mesaj4.out";
const int MaxN=100010;

ifstream fin(InFile);
ofstream fout(OutFile);

vector<int> a[MaxN];
int N,M,x,y;
int viz[MaxN];

void dfs(int nod)
{
    viz[nod]=1;
    for(register int i=0;i<(int)a[nod].size();++i)
    {
        if(viz[a[nod][i]]==0)
        {
            dfs(a[nod][i]);
            fout<<a[nod][i]<<" "<<nod<<"\n";
        }
    }
}

void bfs(int nod)
{
    queue<int> q;
    q.push(nod);
    viz[nod]=0;
    while(!q.empty())
    {
        nod=q.front();
        q.pop();
        for(register int i=0;i<(int)a[nod].size();++i)
        {
            if(viz[a[nod][i]]==1)
            {
                q.push(a[nod][i]);
                viz[a[nod][i]]=0;
                fout<<nod<<" "<<a[nod][i]<<"\n";
            }
        }
    }
}

int bfs2(int nod)
{
    queue<int> q;
    q.push(nod);
    viz[nod]=1;
	int nr=0;
    while(!q.empty())
    {
        nod=q.front();
        q.pop();
		++nr;
        for(register int i=0;i<(int)a[nod].size();++i)
        {
            if(viz[a[nod][i]]==0)
            {
                q.push(a[nod][i]);
                viz[a[nod][i]]=1;
            }
        }
    }
	return nr;
}


int main()
{
    fin>>N>>M;
    for(register int i=0;i<M;++i)
    {
        fin>>x>>y;
        a[x].push_back(y);
        a[y].push_back(x);
    }
    fin.close();

	if(bfs2(1)==N)
	{
		for(register int i=0;i<=N;++i)
		{
			viz[i]=0;
		}

	    fout<<(N-1)*2<<"\n";
	    dfs(1);
	    bfs(1);
	}
	else
	{
		fout<<"-1";
	}
    fout.close();
    return 0;
}