#include <bits/stdc++.h>

#define MAXN 100005

int N, M, P[MAXN], Perm[MAXN];
std::vector <int> ADC[MAXN], TADC[MAXN];
std::vector <int> Leaves;

inline void AddEdge(int X, int Y) {
    ADC[X].push_back(Y);
    ADC[Y].push_back(X);
}

int Reach, LVL[MAXN];
std::vector <std::pair <int, int>> Pairs;
void DFS(int Vertex = 1, int Level = 1) {
    ++ Reach;
    LVL[Vertex] = Level;
    for (auto Edge:ADC[Vertex])
        if (!LVL[Edge])
            DFS(Edge, Level+1), Pairs.push_back({Edge, Vertex});
}

bool cmp(int idx, int idy) {
    return LVL[idx] < LVL[idy];
}

std::ifstream In ("mesaj4.in");
std::ofstream Out("mesaj4.out");

void Print() {  Out << 2*N - 2 << '\n';
    for (int i=0; i<Pairs.size(); ++i)
        Out << Pairs[i].first << ' ' << Pairs[i].second << '\n';
    for (int i=Pairs.size()-1; i>=0; --i)
        Out << Pairs[i].second << ' ' << Pairs[i].first << '\n';
}

void Citire() {
    In >> N >> M;
    for (int i=1, X, Y; i<=M; ++i)
        In >> X >> Y, AddEdge(X, Y);
}

void Rezolvare() {
    DFS();
    if (Pairs.size() == N-1)
        Print();
    else
        Out << "-1\n";
}

int main()
{
    Citire();
    Rezolvare();

    return 0;
}