import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.NoSuchElementException;

public class Main {
	final long MOD=(long)1e9+7;
	
	long ADD(long a,long b) {
		return a+b>=MOD?a+b-MOD:a+b;
	}
	
	//t[i]==1 => p[i]>=X[i] :up
	//t[i]==0 => p[i]<=X[i] :dn
	
	void run() {
		FastScanner sc=new FastScanner();
		PrintWriter pw=new PrintWriter(System.out);
		int N=sc.nextInt();//1<=N<=3000
		int[] t=new int[N];
		int[] X=new int[N];
		ArrayList<Integer> dn=new ArrayList<>();
		ArrayList<Integer> up=new ArrayList<>();
		for (int i=0;i<N;++i) {
			t[i]=sc.nextInt();
			X[i]=sc.nextInt();//1<=X[i]<=N
			--X[i];
			if (t[i]==1) {
				up.add(X[i]);
			} else {
				dn.add(X[i]);
			}
		}
		long[][] dp=new long[N+1][N+1];//i,j=i個確定、j個upの方に詰めた
		dp[0][0]=1;
		Collections.sort(up);
		Collections.sort(dn,Comparator.comparing(v->-v));
		int p=0,q=dn.size();
		for (int num=0;num<N;++num) { // i を up,dnのどちらかに割り振る
			while(p<up.size()&&up.get(p)<=num) ++p;
			while(q>0&&!(dn.get(q-1)>=num)) --q;
			for (int ups=0;ups<=num;++ups) {
				if (dp[num][ups]==0) continue;
				int dns=num-ups;
				if (p-ups>0) {
					dp[num+1][ups+1]=ADD(dp[num+1][ups+1],dp[num][ups]*(p-ups)%MOD);
				}
				if (q-(dn.size()-dns-1)>0) {
					dp[num+1][ups+0]=ADD(dp[num+1][ups+0],dp[num][ups]*(q-(dn.size()-dns-1))%MOD);
				}
			}
		}
		pw.println(dp[N][up.size()]);
		pw.close();
	}
	
	void tr(Object...objects) {System.out.println(Arrays.deepToString(objects));}
	
    public static void main(String[] args) {
    	new Main().run();
    }
}




class FastScanner {
    private final InputStream in = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;
    private boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        }else{
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }
    private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1;}
    private static boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}
    private void skipUnprintable() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;}
    public boolean hasNext() { skipUnprintable(); return hasNextByte();}
    public String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while(isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
    public long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while(true){
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            }else if(b == -1 || !isPrintableChar(b)){
                return minus ? -n : n;
            }else{
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
    public int nextInt() {
    	return (int)nextLong();
    }
}