#include<fstream>
#include<vector>
#include<iostream>

using namespace std;

class InParser {
private:
    FILE *fin;
    char *buff;
    int sp;

    char read_ch() {
        ++sp;
        if (sp == 4096) {
            sp = 0;
            fread(buff, 1, 4096, fin);
        }
        return buff[sp];
    }

public:
    InParser(const char* nume) {
        fin = fopen(nume, "r");
        buff = new char[4096];
        sp = 4095;
    }

    InParser& operator >> (int &n) {
        char c;
        while (!isdigit(c = read_ch()) && c != '-');
        int sgn = 1;
        if (c == '-') {
            n = 0;
            sgn = -1;
        } else {
            n = c - '0';
        }
        while (isdigit(c = read_ch())) {
            n = 10 * n + c - '0';
        }
        n *= sgn;
        return *this;
    }

    InParser& operator >> (long long &n) {
        char c;
        n = 0;
        while (!isdigit(c = read_ch()) && c != '-');
        long long sgn = 1;
        if (c == '-') {
            n = 0;
            sgn = -1;
        } else {
            n = c - '0';
        }
        while (isdigit(c = read_ch())) {
            n = 10 * n + c - '0';
        }
        n *= sgn;
        return *this;
    }
};

vector<vector<int>>v;
vector<bool>ap;
vector<pair<int,int>>pereche;

inline void DFS(int start)
{
    ap[start] = true;
    for (int i = 0; i < v[start].size(); i++)
    {
        if(ap[v[start][i]] == false)
        {
            pereche.push_back({start, v[start][i]});
            DFS(v[start][i]);
        }
    }
}

int main()
{
    InParser f_in("mesaj4.in");
    ofstream fout("mesaj4.out");
    int X, U;
    f_in >> X >> U;
    v.resize(X);
    ap.resize(X);
    for(int i = 0; i < U ; i++)
    {
        int V1, V2;
        f_in >> V1 >> V2;
        cout << V1 << ' ' << V2 << '\n';
        V1--, V2--;
        v[V1].push_back(V2);
        v[V2].push_back(V1);
    }
    DFS(false);
    if(pereche.size() == X - 1)
    {
        fout << 2 * X - 2 << '\n';
        for(int i = X - 2; i >= 0; i--)
        {
            fout << pereche[i].second + 1 << ' ' << pereche[i].first + 1 << '\n';
        }
        for(int i = 0; i < X - 1; i++)
        {
            fout << pereche[i].first + 1 <<  " " << pereche[i].second + 1 << '\n';
        }
    }
    else
    {
        fout << -1;
    }
    return false;
}