#include<bits/stdc++.h>
using namespace std;
#define int long long

const int N=3005,mod=1e9+7;
int n,s1[N],s2[N],c2[N],fac[N];
int f[N],g[N],C[N][N];

signed main(){
	ios::sync_with_stdio(0);cin.tie(0);
	cin>>n;
	C[0][0]=fac[0]=1;
	for(int i=1;i<=n;i++){
		C[i][0]=1;fac[i]=fac[i-1]*i%mod;
		for(int j=1;j<=i;j++){
			C[i][j]=(C[i-1][j-1]+C[i-1][j])%mod;
		}
	}
	for(int i=1,op,x;i<=n;i++){
		cin>>op>>x;
		if(op==1) s1[x]++;
		else s2[x]++,c2[x]++;
	}
	for(int i=1;i<=n;i++) s1[i]+=s1[i-1],s2[i]+=s2[i-1];
	f[0]=1;
	for(int i=1;i<=n;i++){
		for(int j=0;j<=i;j++) g[j]=f[j],f[j]=0;
		for(int j=0;j<=i;j++){
			(f[j]+=g[j]*(s1[i]-((i-1-j)-s2[i-1])))%=mod;
			(f[j+1]+=g[j])%=mod;
		}
		for(int j=0;j<=i;j++) g[j]=f[j],f[j]=0;
		for(int j=c2[i];j<=i;j++){
			(f[j-c2[i]]+=g[j]*C[j][c2[i]]%mod*fac[c2[i]]%mod)%=mod;
		}
	}
	cout<<f[0];
	return 0;
}