#include <bits/stdc++.h>
#define maxN 100005
using namespace std;
vector<int> v[maxN];
vector<pair<int,int> >sol;
int n,m,i,j,x,y;
bool seen[maxN];
void dfs(int nod)
{
    seen[nod]=true;
    for(vector<int>::iterator it=v[nod].begin();it!=v[nod].end();it++)
        if(!seen[*it])
            dfs(*it),
            sol.push_back(make_pair(*it,nod));
}
int main()
{
    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),
        v[x].push_back(y),
        v[y].push_back(x);
    dfs(1);
    for(i=1;i<=n;i++)
        if(!seen[i])
        {
            printf("-1");
            return 0;
        }
    printf("%d\n",2*sol.size());
    for(int i=0;i<(int)sol.size();i++)
        printf("%d %d\n",sol[i].first,sol[i].second);
    for(int i=sol.size()-1;i>=0;i--)
        printf("%d %d\n",sol[i].second,sol[i].first);
    return 0;
}