#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
 
struct data
{
	int p, q;
	data (int x, int y)
	{
		p = x;
		q = y;
	}
} typedef data;
 
int main ()
{
	vector <data> v;
	int x, y, a, b;
	cin>>x>>y>>a>>b;
	for (int i=a; i<=x; i++)
	{
		for (int j=b; j<i && j<=y; j++)
		{
			v.push_back(data(i, j));
		}
	}
	cout<<v.size()<<endl;
	for (int i=0; i<v.size(); i++)
	{
		cout<<v[i].p<<" "<<v[i].q<<endl;
	}
	return 0;
}