#include <iostream>
#include <cstdio>
#include <vector>
#define MAXN 100050

using namespace std;

int n, m;
vector<int> graf[MAXN];
int parent[MAXN];
vector<int> arb[MAXN];

void read()
{
    int x, y;
    scanf("%d %d", &n, &m);
    for (int i = 1; i <= m; i++) {
        scanf("%d %d", &x, &y);
        graf[x].push_back(y);
        graf[y].push_back(x);
    }
}

void dfs(int nod)
{
    for (int y : graf[nod])
    {
        if (parent[y] != 0) continue;
        parent[y] = nod;
        arb[nod].push_back(y);
        dfs(y);
    }
}

int connected()
{
    for (int i = 1; i <= n; i++)
        if (parent[i] == 0)
            return 0;
    return 1;
}

void downTopSolve(int nod)
{
    for (int y : arb[nod])
        downTopSolve(y);
    if (parent[nod] != -1)
        printf("%d %d\n", nod, parent[nod]);
}

void topDownSolve(int nod)
{
    for (int y : arb[nod])
        printf("%d %d\n", nod, y);
    for (int y : arb[nod])
        topDownSolve(y);
}

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

    read();
    parent[1] = -1;
    dfs(1);
    if (!connected())
        printf("-1");
    else {
        printf("%d\n", 2*(n-1));
        downTopSolve(1);
        topDownSolve(1);
    }

    return 0;
}