#include<stdio.h>
int n;
int l[8], r[8];
int ans;
void check(int p[])
{
	int x = -1, i;
	for (i = 0; i < n; i++)
	{
		if (x < l[p[i]])
			x = l[p[i]];
		else if (x > r[p[i]])
			return;
	}
	ans++;
	return;
}
void make(int a[], int aa, int b[], int bb)
{
	if (bb == 0)
	{
		check(a);
		return;
	}
	int i, c[8], j;
	for (i = 0; i < bb; i++)
	{
		for (j = 0; j < i; j++)
			c[j] = b[j];
		for (j = i + 1; j < bb; j++)
			c[j - 1] = b[j];
		a[aa] = b[i];
		make(a, aa + 1, c, bb - 1);
	}
	return;
}
int main()
{
	scanf("%d", &n);
	int i;
	for (i = 0; i < n; i++)
		scanf("%d %d", &l[i], &r[i]);
	ans = 0;
	int a[8], b[8];
	for (i = 0; i < n; i++)
		b[i] = i;
	make(a, 0, b, n);
	printf("%d\n", ans);
	return 0;
}