#include <iostream>
#include <cstdio>
#include <vector>
#include <cmath>
using namespace std;
int read()
{
	char c=getchar();
	int f=1,x=0;
	while(c<'0'||c>'9')
	{
		if(c=='-') f=-1;
		c=getchar();
	}
	while(c>='0'&&c<='9')
	{
		x=(x<<1)+(x<<3)+(c^'0');
		c=getchar();
	}
	return x*f;
}
void print(int x)
{
	if(x<0)
	{
		putchar('-');
		x=-x;
	}
	if(x>9) print(x/10);
	putchar(x%10+'0');
}
const int N=3e3+5,mod=1e9+7;
int n,a[N],b[N],f[N][N];
int main()
{
//	freopen(".in","r",stdin);
//	freopen(".out","w",stdout);
	n=read();
	for(int i=1;i<=n;i++)
	{
		int o=read();
		if(o) b[read()]++;
		else a[read()]++;
	}
	for(int i=n;i>=1;i--) a[i]+=a[i+1];
	for(int i=1;i<=n;i++) b[i]+=b[i-1];
	int x=a[1];
	f[0][0]=1;
	for(int i=1;i<=n;i++)
	{
		for(int j=0;j<i&&j<=x;j++)
		{
			int val=f[i-1][j];
			if(!val) continue;
			f[i][j+1]+=1ll*val*(a[i]-(x-(j+1)))%mod,f[i][j+1]%=mod;
			f[i][j]+=1ll*val*(b[i]-(i-1-j))%mod,f[i][j]%=mod;
		}
	}
	print(f[n][x]);
	return 0;
}