//Author: Kevin
#include<bits/stdc++.h>
//#pragma GCC optimize("O2")
using namespace std;
#define ll long long
#define ull unsigned ll
#define pb emplace_back
#define mp make_pair
#define ALL(x) (x).begin(),(x).end()
#define rALL(x) (x).rbegin(),(x).rend()
#define srt(x) sort(ALL(x))
#define rev(x) reverse(ALL(x))
#define rsrt(x) sort(rALL(x))
#define sz(x) (int)(x.size())
#define inf 0x3f3f3f3f
#define pii pair<int,int>
#define lb(v,x) (int)(lower_bound(ALL(v),x)-v.begin())
#define ub(v,x) (int)(upper_bound(ALL(v),x)-v.begin())
#define uni(v) v.resize(unique(ALL(v))-v.begin())
#define longer __int128_t
void die(string S){puts(S.c_str());exit(0);}
int A[1010][1010],B[1010][1010];
vector<array<int,4>> vec;
void op(int a,int b,int c,int d)
{
	assert(A[a][b]);
	assert(A[c][d]);
	assert(!A[a][c]);
	assert(!A[b][d]);
	A[a][b]=A[c][d]=A[b][a]=A[d][c]=0;
	A[a][c]=A[b][d]=A[c][a]=A[d][b]=1;
	vec.push_back({a,b,c,d});
}
int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	int n,m;
	cin>>n>>m;
	for(int i=1;i<=m;i++)
	{
		int u,v;
		cin>>u>>v;
		A[u][v]=A[v][u]=1;
	}
	for(int i=1;i<=m;i++)
	{
		int u,v;
		cin>>u>>v;
		B[u][v]=B[v][u]=1;
	}
	for(int i=1;i<=n;i++)
		if(accumulate(A[i]+1,A[i]+n+1,0)!=accumulate(B[i]+1,B[i]+n+1,0))
			die("-1");
	for(int i=1;i<=n;i++)
	{
		for(int j=i+1;j<=n;j++)
			if(A[i][j]&&!B[i][j])
			{
				int p=i+1;
				while(true)
				{
					if(p>n) die("-1");
					while(p<n&&(A[i][p]||!B[i][p])) p++;
					int ok=0;
					for(int y=1;y<=n;y++)
						if(y!=j&&A[y][p]&&!A[j][y])
						{
							op(i,j,p,y);
							ok=1;
							break;
						}
					if(ok) break;
					p++;
				}
			}
	}
	cout<<sz(vec)<<'\n';
	for(auto arr:vec)
	{
		for(auto x:arr)
			cout<<x<<" ";
		cout<<'\n';
	}
	return 0;
}