#include<bits/stdc++.h>
namespace ifzw{
#define ll long long 
#define dd double 
#define ull unsigned ll
#define LL __int128
#define siz(A) ((int)A.size())
using namespace std;
char gc(){static char buf[1<<16],*s,*t;if(s==t){t=(s=buf)+fread(buf,1,1<<16,stdin);if(s==t)return EOF;}return *s++;}
#define getchar gc
ll read()
{
	char c;
	ll w=1;
	while((c=getchar())>'9'||c<'0')if(c=='-')w=-1;
	ll ans=c-'0';
	while((c=getchar())>='0'&&c<='9')ans=(ans<<1)+(ans<<3)+c-'0';
	return ans*w;
}
void pc(char c,int op)
{
	static char buf[1<<16],*s=buf,*t=(buf+(1<<16));
	(op||((*s++=c)&&(s==t)))&&(fwrite(buf,1,s-buf,stdout),s=buf);
}
void wt(int x)
{
	if(x>9)wt(x/10);
	pc('0'+x%10,0);
}
void wts(int x,char op)
{
	if(x<0)pc('-',0),x=-x;
	wt(x),pc(op,0);
}
char ST;
int n,m;
namespace bf
{
	map<array<int,10>,array<int,4> >mp;
	array<int,10>A,B;
	int id[10][10];
	int&get(array<int,10>&C,int x,int y)
	{
		if(x>y)swap(x,y);
		return C[id[x][y]];
	}
	void solve()
	{
		int tt=0;
		for(int i=1;i<=n;i++)
			for(int j=i;j<=n;j++)
				id[i][j]=tt++;
		for(int i=1;i<=m;i++)
		{
			int a=read(),b=read();
			get(A,a,b)=1;
		}
		for(int i=1;i<=m;i++)
		{
			int a=read(),b=read();
			get(B,a,b)=1;
		}
		auto D=A;
		queue<array<int,10>>q;
		q.push(A);
		while(!q.empty())
		{
			A=q.front();q.pop();
			for(int x=1;x<=n;x++)
			{
				for(int y=1;y<=n;y++)
				{
					for(int a=1;a<=n;a++)
					{
						for(int b=1;b<=n;b++)
						{
							if(x==y||x==a||x==b||y==a||y==b||a==b)continue;
							if(get(A,x,y)&&get(A,a,b)&&!get(A,x,a)&&!get(A,y,b))
							{
								get(A,x,y)^=1;
								get(A,a,b)^=1;
								get(A,x,a)^=1;
								get(A,y,b)^=1;
								if(!mp.count(A))q.push(A),mp[A]={x,y,a,b};
								get(A,x,y)^=1;
								get(A,a,b)^=1;
								get(A,x,a)^=1;
								get(A,y,b)^=1;
							}
						}
					}
				}
			}
		}
		//~ cerr<<"!!\n";
		//~ for(int i=0;i<siz(A);i++)cerr<<D[i]<<" ";
		//~ puts("");
		//~ for(int i=0;i<siz(A);i++)cerr<<B[i]<<" ";
		//~ puts("");
		if(!mp.count(B))
		{
			puts("-1");
			exit(0);
		}
		else 
		{
			vector<array<int,4> >ans;
			A=D;
			while(B!=A)
			{
				auto to=mp[B];
				ans.push_back(to);
				int x=to[0],y=to[1],a=to[2],b=to[3];
				//~ cerr<<get(B,x,y)<<" "<<get(B,a,b)<<" "<<get(B,x,a)<<" "<<get(B,y,b)<<"#\n";
				get(B,x,y)^=1;
				get(B,a,b)^=1;
				get(B,x,a)^=1;
				get(B,y,b)^=1;
			}
			//~ cerr<<"!!\n";
			//~ for(int i=0;i<siz(A);i++)cerr<<D[i]<<" ";
			//~ puts("");
			//~ for(int i=0;i<siz(A);i++)cerr<<B[i]<<" ";
			//~ puts("");
			//~ assert(A==B);
			reverse(ans.begin(),ans.end());
			cout<<siz(ans)<<"\n";
			for(auto it:ans)
			{
				for(auto i:it)cout<<i<<" ";
				puts("");
			}
		}
	}
}
namespace zj
{
	random_device R;
	mt19937 G(R());
	int rd(int l,int r){return uniform_int_distribution<int>(l,r)(G);};
	const int xx=1005;
	bitset<xx>fir[xx],sec[xx];
	//~ bitset<xx>xf[xx],xs[xx];
	vector<array<int,4> >ans;
	int d1[xx],d2[xx];
	vector<int>rem;
	void app(int x,int y,int a,int b)
	{
		assert(fir[x][y]&&fir[a][b]&&!fir[x][a]&&!fir[y][b]);
		fir[x][y]=0;
		fir[a][b]=0;
		fir[x][a]=1;
		fir[y][b]=1;
		
		fir[y][x]=0;
		fir[b][a]=0;
		fir[a][x]=1;
		fir[b][y]=1;
		ans.push_back({x,y,a,b});
	}
	bool ck(int x)
	{
		if((fir[x]^sec[x]).any())return 1;
		return 0;
	}
	void solve()
	{
		for(int i=1;i<=m;i++)
		{
			int a=read(),b=read();
			fir[a][b]=fir[b][a]=1;
			d1[a]++,d1[b]++;
		}
		for(int i=1;i<=m;i++)
		{
			int a=read(),b=read();
			sec[a][b]=sec[b][a]=1;
			d2[a]++,d2[b]++;
		}
		int cr=1;
		for(int i=1;i<=n;i++)
			if(d1[i]!=d2[i])cr=0;
		if(!cr)
		{
			puts("-1");
			exit(0);
		}
		for(int i=1;i<=n;i++)
			if(ck(i))rem.push_back(i);
		
		while(siz(rem))
		{
			int ty=rem[rd(0,siz(rem)-1)];
			vector<int>L,R;
			for(int i=1;i<=n;i++)
			{
				if(fir[ty][i]&&!sec[ty][i])L.push_back(i);
				if(!fir[ty][i]&&sec[ty][i])R.push_back(i);
			}
			int turn=100;
			while(siz(L)&&(turn--))
			{
				int to=rd(1,n),A=L[rd(0,siz(L)-1)],B=R[rd(0,siz(R)-1)];
				if(to==A||to==B||to==ty)
				{
					++turn;
					continue;
				}
				if(!fir[to][A]&&fir[to][B])
				{
					//keyi zuo
					if(sec[to][A]||!sec[to][B])
					{
						int F=ck(to);
						app(ty,A,B,to);
						L.erase(find(L.begin(),L.end(),A));
						R.erase(find(R.begin(),R.end(),B));
						int S=ck(to);
						if(F&&!S)rem.erase(find(rem.begin(),rem.end(),to));
						if(!F&&S)rem.push_back(to);
					}
				}
			}
			if(!ck(ty))rem.erase(find(rem.begin(),rem.end(),ty));
		}
		
		cout<<siz(ans)<<"\n";
		for(auto its:ans)
		{
			for(auto it:its)cout<<it<<" ";
			puts("");
		}
	}
}
char ED;
int main(){
	cerr<<abs(&ST-&ED)/1024.0/1024<<"%\n";
	freopen("graph.in","r",stdin);
	freopen("graph.out","w",stdout);
	n=read(),m=read();
	//~ if(n<=5)
	//~ {
		//~ bf::solve();
		//~ return 0;
	//~ }
	zj::solve();
	pc('1',1);
	return 0;
}

}signed main(){return ifzw::main();}