#include <bits/stdc++.h>

using namespace std;

const int MAXN = 1e5 + 5;

FILE *fin, *fout;

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

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

void DFS( int node ) {
    seen[node] = 1;

    for (int son: G[node])
        if (!seen[son]) {
            height[son] = height[node] + 1;
            h[height[son]].push_back(son);
            father[son] = node;
            answer[ans++] = {node, son};
            DFS(son);
        }
}

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

    return 1;
}

void BFS() {
    for (int i = n; i > 1; i--) {
        int node;
        while(!h[i].empty()) {
            node = h[i].back();
            h[i].pop_back();
            answer[ans++] = {node, father[node]};
        }
    }
}

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);
    }

    height[1] = 1;
    h[1].push_back(1);

    DFS( 1 );

    if (check()) {
        BFS();

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

    fclose( fin );
    fclose( fout );

    return 0;
}