#include <fstream>
#include <vector>

using namespace std;

ifstream fin("mesaj4.in");
ofstream fout("mesaj4.out");

const int DIM = 100010;

int n, m, x, y;
vector<int> l[DIM];
vector<pair<int, int>> sol;
bool visited[DIM];

void dfs(int node) {
    visited[node] = true;
    for (auto adjNode : l[node]) {
        if (!visited[adjNode]) {
            dfs(adjNode);
            sol.emplace_back(adjNode, node);
        }
    }
}

int main() {
    fin >> n >> m;
    for (int i = 1; i <= m; i++) {
        fin >> x >> y;
        l[x].emplace_back(y);
        l[y].emplace_back(x);
    }

    dfs(1);
    for (int node = 1; node <= n; node++) {
        if (!visited[node]) {
            fout << -1;
            return 0;
        }
    }


    fout << 2 * sol.size() << '\n';
    for (auto elem = sol.begin(); elem != sol.end(); elem++)
        fout << elem->first << ' ' << elem->second << '\n';
    for (auto elem = sol.rbegin(); elem != sol.rend(); elem++)
        fout << elem->second << ' ' << elem->first << '\n';

    return 0;
}