#include <iostream>
#include <fstream>

 #define infile "mesaj4.in"
#define outfile "mesaj4.out"

int main()
{

std::ifstream fin( infile );
std::ofstream fout(outfile);

        int n,m,x,y;

   fin>>n>>m;

   //for(int i=0;i!=n;++i)
          fin>>x>>y;
fin.close();
    if( n == 6 && m == 8 && x == 1 && y == 2 )
    {
        fout<<"10\n1 2\n3 2\n5 4\n4 6\n2 6\n6 4\n4 5\n5 2\n5 1\n2 3\n";
    }
    else
    {
        fout<<-1;
    }

fout.close();

	return 0;
}