//horatiu11
# include <cstdio>
# include <vector>
# define nmax 100001
using namespace std;
int n,m,x,y,nr;
bool viz[nmax];
vector <int>G[nmax];
struct muchie{int x,y;}e;
vector <muchie>sol;
void dfs(int x)
{
    vector <int>::iterator it;
    viz[x]=true;
    for(it=G[x].begin();it!=G[x].end();++it)
        if(!viz[*it])
        {
            dfs(*it);
            e.x=*it;e.y=x;++nr;
            sol.push_back(e);
        }
}
int main()
{
    int i;
    freopen("mesaj4.in","r",stdin);
    freopen("mesaj4.out","w",stdout);
    scanf("%d%d",&n,&m);
    for(i=1;i<=m;++i)
    {
        scanf("%d%d",&x,&y);
        G[x].push_back(y);
        G[y].push_back(x);
    }
    dfs(1);
    if(nr!=n-1)printf("-1\n");
    else
    {
        vector <muchie>::iterator it;
        printf("%d\n",2*(n-1));
        for(it=sol.begin();it!=sol.end();++it)
            printf("%d %d\n",(*it).x,(*it).y);
        for(it=sol.end()-1;;--it)
        {
            printf("%d %d\n",(*it).y,(*it).x);
            if(it==sol.begin())break;
        }
    }
    return 0;
}