#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],vx[MaxN],vy[MaxN],vind,nr;

void dfs(int nod)
{
    viz[nod]=1;
	++nr;
    for(register int i=0;i<(int)a[nod].size();++i)
    {
        if(viz[a[nod][i]]==0)
        {
            dfs(a[nod][i]);
			++vind;
            vx[vind]=a[nod][i];
			vy[vind]=nod;
        }
    }
}

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;
				++vind;
                vx[vind]=nod;
				vy[vind]=a[nod][i];
            }
        }
    }
}

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();

	dfs(1);

	if(nr==N)
	{
		bfs(1);
		fout<<(N-1)*2<<"\n";
		for(register int i=1;i<=vind;++i)
		{
			fout<<vx[i]<<" "<<vy[i]<<"\n";
		}
	}
	else
	{
		fout<<"-1";
	}
    fout.close();
    return 0;
}