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

typedef pair<int, int> pii;
typedef long long ll;
template<class T> using V = vector<T>;
template<class T> using VV = V<V<T>>;

#define pb push_back
#define eb emplace_back
#define mp make_pair
#define fi first
#define se second
#define rep(i,n) rep2(i,0,n)
#define rep2(i,m,n) for(int i=m;i<(n);i++)
#define ALL(c) (c).begin(),(c).end()

#ifdef LOCAL
#define dump(x) cerr << __LINE__ << " " << #x << " = " << (x) << endl
#else 
#define dump(x) true
#endif

constexpr ll TEN(int n) { return (n == 0) ? 1 : 10 * TEN(n-1); }

template<class T, class U> void chmin(T& t, const U& u) { if (t > u) t = u; }
template<class T, class U> void chmax(T& t, const U& u) { if (t < u) t = u; }

template<class T, class U>
ostream& operator<<(ostream& os, const pair<T, U>& p) {
	os<<"("<<p.first<<","<<p.second<<")";
	return os;
}

template<class T>
ostream& operator<<(ostream& os, const vector<T>& v) {
	os<<"{";
	rep(i, v.size()) {
		if (i) os<<",";
		os<<v[i];
	}
	os<<"}";
	return os;
}

template <unsigned int MOD>
struct ModInt {
	using uint = unsigned int;
	using ull = unsigned long long;
	using M = ModInt;

	uint v;

	ModInt(ll _v = 0) { set_norm(_v % MOD + MOD); }
	M& set_norm(uint _v) { //[0, MOD * 2)->[0, MOD)
		v = (_v < MOD) ? _v : _v - MOD;
		return *this;
	}

	explicit operator bool() const { return v != 0; }
	M operator+(const M& a) const { return M().set_norm(v + a.v); }
	M operator-(const M& a) const { return M().set_norm(v + MOD - a.v); }
	M operator*(const M& a) const { return M().set_norm(ull(v) * a.v % MOD); }
	M operator/(const M& a) const { return *this * a.inv(); }
	M& operator+=(const M& a) { return *this = *this + a; }
	M& operator-=(const M& a) { return *this = *this - a; }
	M& operator*=(const M& a) { return *this = *this * a; }
	M& operator/=(const M& a) { return *this = *this / a; }
	M operator-() const { return M() - *this; }
	M& operator++(int) { return *this = *this + 1; }
	M& operator--(int) { return *this = *this - 1; }

	M pow(ll n) const {
		if (n < 0) return inv().pow(-n);
		M x = *this, res = 1;
		while (n) {
			if (n & 1) res *= x;
			x *= x;
			n >>= 1;
		}
		return res;
	}

	M inv() const {
		ll a = v, b = MOD, p = 1, q = 0, t;
		while (b != 0) {
			t = a / b;
			swap(a -= t * b, b);
			swap(p -= t * q, q);
		}
		return M(p);
	}

	bool operator==(const M& a) const { return v == a.v; }
	bool operator!=(const M& a) const { return v != a.v; }
	friend ostream& operator<<(ostream& os, const M& a) { return os << a.v; }
	static int get_mod() { return MOD; }	
};

using Mint = ModInt<1000000007>;

const int maxv = 1010;

Mint fact[maxv], finv[maxv], inv[maxv], p3[maxv];

void init() {
    fact[0] = 1;
    p3[0] = 1;
    for (int i = 1; i < maxv; ++i) {
        fact[i] = fact[i-1] * i;
        p3[i] = p3[i-1] * 3;
    }

    finv[maxv-1] = fact[maxv-1].inv();
    for (int i = maxv - 2; i >= 0; --i) {
        finv[i] = finv[i+1] * (i+1);
    }

    for (int i = 1; i < maxv; ++i) {
        inv[i] = finv[i] * fact[i-1];
    }
}

Mint comb(int n, int r) {
    if (n < 0 || r < 0 || r > n) return Mint(0);
    return fact[n] * finv[r] * finv[n-r];
}

const int maxn = 310;

Mint dp[maxn * 2][maxn][maxn];
int state[maxn * 2];

int main() {
	init();

	int N; cin >> N;
	V<int> A(N * 2);

	rep(i, N * 2) {
		cin >> A[i];
	}

	int free = 0;

	rep(i, N) {
		int p = i * 2, q = i * 2 + 1;
		if (A[p] < A[q]) swap(A[p], A[q]);
		if (A[q] != -1) {
			state[A[p]] = state[A[q]] = -1;
		} else if (A[p] != -1) {
			state[A[p]] = 1;
		} else if (A[p] == -1) {
			free++;
		}
	}

	dp[N * 2][0][0] = 1;
	for (int i = N*2; i >= 1; --i) {
		if (state[i] == -1) {
			for (int j = 0; j <= N; ++j) {
				for (int k = 0; k <= N; ++k) {
					dp[i-1][j][k] += dp[i][j][k];
				}
			}
		} else if (state[i] == 1) {
			for (int j = 0; j <= N; ++j) {
				for (int k = 0; k <= N; ++k) {
					if (dp[i][j][k]) {
						if (j) {
							dp[i-1][j-1][k] += dp[i][j][k];
						}
						dp[i-1][j][k+1] += dp[i][j][k];
					}
				}
			}			
		} else {
			for (int j = 0; j <= N; ++j) {
				for (int k = 0; k <= N; ++k) {
					if (dp[i][j][k]) {
						dp[i-1][j+1][k] += dp[i][j][k];
						if (j) {
							dp[i-1][j-1][k] += dp[i][j][k];
						}
						if (k) {
							dp[i-1][j][k-1] += dp[i][j][k] * k;
						}
					}
				}
			}			
		}
	}

	Mint ans = dp[0][0][0];
	for (int i = 1; i <= free; ++i) {
		ans = ans * i;
	}
	cout << ans << endl;

	return 0;
}

Measured memory_bytes: 232960