#include <fstream>
#include <vector>
#include <algorithm>

using namespace std;

ifstream in("mesaj4.in");
ofstream out("mesaj4.out");

const int NMAX = 100000;

struct DAP {
    int x,y;
};

DAP make_DAP( int x, int y ) {
    DAP S;
    S.x = x;
    S.y = y;
    return S;
}

vector <int> G[NMAX+1];
vector <DAP> sol;
int N,M, vizitati = 0;
bool viz[NMAX+1];

void DFS( int nod ) {
    ++vizitati;
    viz[nod] = 1;

    for( int i = 0;  i < (int)G[nod].size();  ++i ) {
        int x = G[nod][i];
        if( !viz[x] ) {
            sol.push_back( make_DAP(x, nod) );
            DFS( x );
        }
    }
}

int main() {
    in >> N >> M;
    for( int i = 1;  i <= M;  ++i ) {
        int x,y;
        in >> x >> y;
        G[x].push_back(y);
        G[y].push_back(x);
    }
    DFS(1);
    if( vizitati < N ) {
        out << "-1\n";
        return 0;
    }
    int sz = (int)sol.size();
    out << 2*sz << '\n';
    for( int i = 0;  i < sz;  ++i ) {
        out << sol[i].x << ' ' << sol[i].y << '\n';
    }
    for( int i = sz-1;  i >= 0;  --i ) {
        out << sol[i].y << ' ' << sol[i].x << '\n';
    }
    return 0;
}