#include <bits/stdc++.h>

using namespace std;

const int MAXN = 1e5 + 5;

FILE *fin, *fout;

vector < int > G[MAXN];
pair < int, int > answer[2 * MAXN];

int k, m, n, ans, father[MAXN];
bool seen[MAXN];

void DFS( int node ) {
    seen[node] = 1;
    for(int son: G[node])
        if(!seen[son]) {
            DFS( son );
            answer[ans++] = {son, node};
        }
}

bool check() {
    for (int i = 1; i <= n; i++)
        if(!seen[i])
            return 0;

    return 1;
}

int main()
{
    fin = fopen( "mesaj4.in", "r" );
    fout= fopen( "mesaj4.out","w" );

    int x, y;

    fscanf( fin, "%d%d", &n, &m );

    for (int i = 1; i <= m; i++) {
        fscanf( fin, "%d%d", &x, &y );
        G[x].push_back(y);
        G[y].push_back(x);
    }

    DFS( 1 );

    if (check()) {
        fprintf( fout, "%d\n", ans*2 );
        for (int i = 0; i < ans; i++)
            fprintf( fout, "%d %d\n", answer[i].first, answer[i].second );
        for (int i = ans-1; i >= 0; i--)
            fprintf( fout, "%d %d\n", answer[i].second, answer[i].first );
    }
    else
        fprintf( fout, "-1" );

    fclose( fin );
    fclose( fout );

    return 0;
}