#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 T[20];
int cnt;
int m;
void dfs(int x)
{
    viz[x] = 1;
    int sz = g[x].size();
    for(int i = 0; i < sz; i++)
    {
        if(!viz[g[x][i]])
        {
            dfs(g[x][i]);
            cnt++;
            rec[cnt] = make_pair(x, g[x][i]);
        }
    }
}

int main()
{
    freopen("mesaj4.in", "r", stdin);
    freopen("mesaj4.out", "w", stdout);
    int n;
    scanf("%d%d", &n, &m);
    gets(T);
    for(int i = 1; i <= m; i++)
    {
        gets(T);
        int p = strlen(T);
        int poz = 0;
        int x = 0;
        for(poz = 0; T[poz] >= '0' && T[poz] <= '9'; poz++)
            x = x * 10 + T[poz] - '0';
        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);
    }
    dfs(1);
    if(cnt < n - 1)
    {
        printf("-1\n");
        return 0;
    }
    printf("%d\n", cnt * 2);
    for(int i = 1; i <= cnt; i++)
        printf("%d %d\n", rec[i].second, rec[i].first);
    for(int i = cnt; i >= 1; i--)
        printf("%d %d\n", rec[i].first, rec[i].second);
    return 0;
}