#include<fstream>
#include<vector>
#include<bitset>

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;
    }
};

class OutParser {
private:
    FILE *fout;
    char *buff;
    int sp;

    void write_ch(char ch) {
        if (sp == 50000) {
            fwrite(buff, 1, 50000, fout);
            sp = 0;
            buff[sp++] = ch;
        } else {
            buff[sp++] = ch;
        }
    }


public:
    OutParser(const char* name) {
        fout = fopen(name, "w");
        buff = new char[50000]();
        sp = 0;
    }
    ~OutParser() {
        fwrite(buff, 1, sp, fout);
        fclose(fout);
    }

    OutParser& operator << (int vu32) {
        if (vu32 <= 9) {
            write_ch(vu32 + '0');
        } else {
            (*this) << (vu32 / 10);
            write_ch(vu32 % 10 + '0');
        }
        return *this;
    }

    OutParser& operator << (long long vu64) {
        if (vu64 <= 9) {
            write_ch(vu64 + '0');
        } else {
            (*this) << (vu64 / 10);
            write_ch(vu64 % 10 + '0');
        }
        return *this;
    }

    OutParser& operator << (char ch) {
        write_ch(ch);
        return *this;
    }
    OutParser& operator << (const char *ch) {
        while (*ch) {
            write_ch(*ch);
            ++ch;
        }
        return *this;
    }
};

InParser fin("mesaj4.in");
OutParser fout("mesaj4.out");

vector<vector<int>>v;
bitset <100000>ap;
vector<pair<int,int>>pereche;

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

int main()
{
    ios_base::sync_with_stdio(false);
    int X, U;
    fin >> X >> U;
    v.resize(X);
    for(int i = 0; i < U ; i++)
    {
        int V1, V2;
        fin >> V1 >> V2;
        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;
}