import java.io.*;
import java.math.*;
import java.util.*;
import java.util.stream.*;

public class Main {

	static final int P = 1_000_000_007;

	void submit() {
		int n = nextInt() * 2;
		int[] type = new int[n];
		int rnd = 0;
		for (int i = 0; i < n / 2; i++) {
			int x = nextInt() - 1;
			int y = nextInt() - 1;
			if (x > y) {
				int tmp = x;
				x = y;
				y = tmp;
			}
			if (x != -2) {
				type[x] = type[y] = 2;
			} else if (y == -2) {
				rnd++;
			} else {
				type[y] = 1;
			}
		}

		int[][] dp = new int[n + 1][n + 1];

		dp[0][0] = 1;

		for (int i = n - 1; i >= 0; i--) {
			int t = type[i];
			if (t == 2) {
				continue;
			}

			int[][] nxt = new int[n + 1][n + 1];
			if (t == 0) {

				for (int j = 0; j <= n; j++) {
					for (int k = 0; k <= n; k++) {
						long val = dp[j][k];
						if (val == 0) {
							continue;
						}
						nxt[j + 1][k] += val;
						if (nxt[j + 1][k] >= P) {
							nxt[j + 1][k] -= P;
						}
						if (j > 0) {
							nxt[j - 1][k] += val;
							if (nxt[j - 1][k] >= P) {
								nxt[j - 1][k] -= P;
							}
						}
						if (k > 0) {
							long delta = (long)val * k % P;
							nxt[j][k - 1] += delta;
							if (nxt[j][k - 1] >= P) {
								nxt[j][k - 1] -= P;
							}
						}
					}
				}
			} else {
				for (int j = 0; j <= n; j++) {
					for (int k = 0; k <= n; k++) {
						long val = dp[j][k];
						if (val == 0) {
							continue;
						}
						if (j > 0) {
							nxt[j - 1][k] += val;
							if (nxt[j - 1][k] >= P) {
								nxt[j - 1][k] -= P;
							}
						}
						nxt[j][k + 1] += val;
						if (nxt[j][k + 1] >= P) {
							nxt[j][k + 1] -= P;
						}
					}
				}
			}
			dp = nxt;
		}
		long ret = dp[0][0];
		for (int i = 1; i <= rnd; i++) {
			ret = ret * i % P;
		}
		out.println(ret);

	}

	void test() {

	}

	void stress() {
		for (int tst = 0;; tst++) {
			if (false) {
				throw new AssertionError();
			}
			System.err.println(tst);
		}
	}

	Main() throws IOException {
		is = System.in;
		out = new PrintWriter(System.out);
		submit();
		// stress();
		// test();
		out.close();
	}

	static final Random rng = new Random();
	static final int C = 5;

	static int rand(int l, int r) {
		return l + rng.nextInt(r - l + 1);
	}

	public static void main(String[] args) throws IOException {
		new Main();
	}

	private InputStream is;
	PrintWriter out;

	private byte[] buf = new byte[1 << 14];
	private int bufSz = 0, bufPtr = 0;

	private int readByte() {
		if (bufSz == -1)
			throw new RuntimeException("Reading past EOF");
		if (bufPtr >= bufSz) {
			bufPtr = 0;
			try {
				bufSz = is.read(buf);
			} catch (IOException e) {
				throw new RuntimeException(e);
			}
			if (bufSz <= 0)
				return -1;
		}
		return buf[bufPtr++];
	}

	private boolean isTrash(int c) {
		return c < 33 || c > 126;
	}

	private int skip() {
		int b;
		while ((b = readByte()) != -1 && isTrash(b))
			;
		return b;
	}

	String nextToken() {
		int b = skip();
		StringBuilder sb = new StringBuilder();
		while (!isTrash(b)) {
			sb.appendCodePoint(b);
			b = readByte();
		}
		return sb.toString();
	}

	String nextString() {
		int b = readByte();
		StringBuilder sb = new StringBuilder();
		while (!isTrash(b) || b == ' ') {
			sb.appendCodePoint(b);
			b = readByte();
		}
		return sb.toString();
	}

	double nextDouble() {
		return Double.parseDouble(nextToken());
	}

	char nextChar() {
		return (char) skip();
	}

	int nextInt() {
		int ret = 0;
		int b = skip();
		if (b != '-' && (b < '0' || b > '9')) {
			throw new InputMismatchException();
		}
		boolean neg = false;
		if (b == '-') {
			neg = true;
			b = readByte();
		}
		while (true) {
			if (b >= '0' && b <= '9') {
				ret = ret * 10 + (b - '0');
			} else {
				if (b != -1 && !isTrash(b)) {
					throw new InputMismatchException();
				}
				return neg ? -ret : ret;
			}
			b = readByte();
		}
	}

	long nextLong() {
		long ret = 0;
		int b = skip();
		if (b != '-' && (b < '0' || b > '9')) {
			throw new InputMismatchException();
		}
		boolean neg = false;
		if (b == '-') {
			neg = true;
			b = readByte();
		}
		while (true) {
			if (b >= '0' && b <= '9') {
				ret = ret * 10 + (b - '0');
			} else {
				if (b != -1 && !isTrash(b)) {
					throw new InputMismatchException();
				}
				return neg ? -ret : ret;
			}
			b = readByte();
		}
	}
}


Measured memory_bytes: 280144