#include <algorithm>
#include <iostream>
#include <cstring>
#include <cstdio>
#include <vector>
#include <bitset>
using namespace std;
const int maxn = 100005;
pair <int, int> rec[maxn];
vector <int> g[maxn];
bitset <maxn> viz;
char afis[maxn * 10];
char T[20];
int ind;
int cnt;
int m;
void dfs(int x)
{
    viz[x] = 1;
    for(auto it : g[x])
    {
        if(!viz[it])
        {
            dfs(it);
            cnt++;
            rec[cnt] = make_pair(x, it);
        }
    }
}

void citire()
{
    gets(T);
    for(int i = 1; i <= m; i++)
    {
        gets(T);
        int p = strlen(T);
        int poz = 0;
        int x = 0;
        while(T[poz] >= '0' && T[poz] <= '9')
        {
            x = x * 10 + T[poz] - '0';
            poz++;
        }
        poz++;
        int y = 0;
        for(int j = poz; j < p; j++)
            y = y * 10 + T[j] - '0';
        g[x].push_back(y);
        g[y].push_back(x);
    }
}

string rast(int n)
{
    string p = "";
    while(n > 0)
    {
        p += (char)(n % 10 + '0');
        n = n / 10;
    }
    return p;
}

inline void adaug(int num)
{
    string s = rast(num);
    reverse(s.begin(), s.end());
    int sz = s.size();
    for(int i = 0; i < sz; i++)
    {
        afis[ind] = s[i];
        ind++;
    }
}

int main()
{
    freopen("mesaj4.in", "r", stdin);
    freopen("mesaj4.out", "w", stdout);
    int n;
    scanf("%d%d", &n, &m);
    citire();
    dfs(1);
    if(cnt < n - 1)
    {
        printf("-1\n");
        return 0;
    }
    printf("%d\n", cnt * 2);
    for(int i = 1; i <= cnt; i++)
    {
        adaug(rec[i].second);
        afis[ind++] = ' ';
        adaug(rec[i].first);
        afis[ind++] = '\n';
    }
    for(int i = cnt; i >= 1; i--)
    {
        adaug(rec[i].first);
        afis[ind++] = ' ';
        adaug(rec[i].second);
        afis[ind++] = '\n';
    }
    printf("%s", afis);
    return 0;
}