#include<cstdio>
#include<vector>

using namespace std;

const int nmax=1e5+5;

vector<int> g[nmax];
bool viz[nmax];
typedef pair<int,int> ii;
vector<ii> vi;
int n,m;

inline void dfs(int node)
{
    viz[node]=1;
    int i,currnode;
    for(i=0;i<g[node].size();++i)
    {
        currnode=g[node][i];
        if(!viz[currnode])
        {
            vi.push_back(ii(node,currnode));
            dfs(currnode);
        }
    }
}

int main()
{
    freopen("mesaj4.in","r",stdin);
    freopen("mesaj4.out","w",stdout);
    int i;
    scanf("%d%d",&n,&m);
    for(i=1;i<=m;++i)
    {
        int x,y;
        scanf("%d%d",&x,&y);
        g[x].push_back(y);
        g[y].push_back(x);
    }
    dfs(1);
    if(vi.size()< n-1)
    {
        printf("-1");
        return 0;
    }
    printf("%d\n",((n-1)<<1));
    for(i=0;i<n-1;++i)
        printf("%d %d\n",vi[i].first,vi[i].second);
    for(i=0;i<n-1;++i)
        printf("%d %d\n",vi[i].second,vi[i].first);
}