#include <cassert>
#include <cstdio>
#include <vector>
using namespace std;

const int MAX_N = 100005;

int n, m;
vector<int> g[MAX_N];
bool vis[MAX_N];
int father[MAX_N];
vector<int> v;

void df(int node) {
    vis[node] = true;

    for (vector<int>::iterator it = g[node].begin(); it != g[node].end(); ++it)
        if (!vis[*it]) {
            father[*it] = node;
            df(*it);
        }

    v.push_back(node);
}

int main() {
    assert(freopen("mesaj4.in", "r", stdin) != NULL);
    assert(freopen("mesaj4.out", "w", stdout) != NULL);

    assert(scanf("%d %d", &n, &m) == 2);
    for (int i = 0; i < m; ++i) {
        int a, b;
        assert(scanf("%d %d", &a, &b) == 2);
        g[a].push_back(b);
        g[b].push_back(a);
    }

    df(1);

    if ((int)v.size() != n) {
        printf("-1\n");
        return 0;
    }

    printf("%d\n", 2 * n - 2);
    for (int i = 0; i + 1 < n; ++i)
        printf("%d %d\n", v[i], father[v[i]]);
    printf("1 1\n");
    for (int i = n - 2; i >= 0; --i)
        printf("%d %d\n", father[v[i]], v[i]);
}