#include<iostream>
#include<string>
#include<cstdio>
#include<vector>
#include<cmath>
#include<algorithm>
#include<functional>
#include<iomanip>
#include<queue>
#include<ciso646>
#include<random>
#include<map>
#include<set>
#include<bitset>
#include<stack>
#include<unordered_map>
#include<utility>
#include<cassert>
#include<complex>
#include<numeric>
using namespace std;

//#define int long long
typedef long long ll;

typedef unsigned long long ul;
typedef unsigned int ui;
constexpr ll mod = 1000000007;
const ll INF = mod * mod;
typedef pair<int, int>P;
#define stop char nyaa;cin>>nyaa;
#define rep(i,n) for(int i=0;i<n;i++)
#define per(i,n) for(int i=n-1;i>=0;i--)
#define Rep(i,sta,n) for(int i=sta;i<n;i++)
#define rep1(i,n) for(int i=1;i<=n;i++)
#define per1(i,n) for(int i=n;i>=1;i--)
#define Rep1(i,sta,n) for(int i=sta;i<=n;i++)
#define all(v) (v).begin(),(v).end()
typedef pair<ll, ll> LP;
typedef long double ld;
typedef pair<ld, ld> LDP;
const ld eps = 1e-12;
const ld pi = acos(-1.0);

ll mod_pow(ll x, ll n) {
	ll res = 1;
	while (n) {
		if (n & 1)res = res * x % mod;
		x = x * x % mod; n >>= 1;
	}
	return res;
}
struct modint {
	ll n;
	modint() :n(0) { ; }
	modint(ll m) :n(m) {
		if (n >= mod)n %= mod;
		else if (n < 0)n = (n % mod + mod) % mod;
	}
	operator int() { return n; }
};
bool operator==(modint a, modint b) { return a.n == b.n; }
modint operator+=(modint& a, modint b) { a.n += b.n; if (a.n >= mod)a.n -= mod; return a; }
modint operator-=(modint& a, modint b) { a.n -= b.n; if (a.n < 0)a.n += mod; return a; }
modint operator*=(modint& a, modint b) { a.n = ((ll)a.n * b.n) % mod; return a; }
modint operator+(modint a, modint b) { return a += b; }
modint operator-(modint a, modint b) { return a -= b; }
modint operator*(modint a, modint b) { return a *= b; }
modint operator^(modint a, int n) {
	if (n == 0)return modint(1);
	modint res = (a * a) ^ (n / 2);
	if (n % 2)res = res * a;
	return res;
}

ll inv(ll a, ll p) {
	return (a == 1 ? 1 : (1 - p * inv(p % a, a)) / a + p);
}
modint operator/(modint a, modint b) { return a * modint(inv(b, mod)); }

const int max_n = 1 << 20;
modint fact[max_n], factinv[max_n];
void init_f() {
	fact[0] = modint(1);
	for (int i = 0; i < max_n - 1; i++) {
		fact[i + 1] = fact[i] * modint(i + 1);
	}
	factinv[max_n - 1] = modint(1) / fact[max_n - 1];
	for (int i = max_n - 2; i >= 0; i--) {
		factinv[i] = factinv[i + 1] * modint(i + 1);
	}
}
modint comb(int a, int b) {
	if (a < 0 || b < 0 || a < b)return 0;
	return fact[a] * factinv[b] * factinv[a - b];
}


//保留してる数、用済みの数
modint dp[303][303];

modint cop[303][303];
void solve() {
	int n; cin >> n;
	vector<int> a(2*n);
	rep(i, 2 * n) {
		cin >> a[i];
	}
	int d = 0;
	vector<int> t(2 * n + 1,0);
	rep(i, n) {
		int le = a[2 * i];
		int ri = a[2 * i + 1];
		if (le < 0 && ri < 0) {
			d++;
		}
		else if (le < 0 || ri < 0) {
			int val = le; if (le < 0)val = ri;
			t[val] = 1;
		}
		else {
			t[le] = t[ri] = 2;
		}
	}

	dp[0][0] = 1;
	rep1(i, 2 * n) {
		if (t[i] == 2)continue;
		else if (t[i] == 1) {
			rep(j, n + 1)rep(k, n + 1)cop[j][k] = 0;
			rep(j, n + 1)rep(k, n + 1) {
				if (dp[j][k] == (modint)0)continue;
				//put
				if (j > 0) {
					cop[j - 1][k] += dp[j][k] * (modint)j;
				}
				//not put
				cop[j][k + 1] += dp[j][k];
			}
			rep(j, n + 1)rep(k, n + 1)dp[j][k] = cop[j][k];
		}
		else {
			rep(j, n + 1)rep(k, n + 1)cop[j][k] = 0;
			int p0 = 0, p1 = 0;
			rep1(j, i - 1) {
				if (t[j] == 0)p0++;
				else if (t[j] == 1)p1++;
			}
			rep(j, n + 1)rep(k, n + 1) {
				if (dp[j][k] == (modint)0)continue;
				int x = k - p1 + p0 - j;
				assert(x % 2 == 0);
				x /= 2;
				x = d - x;
				//put on {-1,-1}
				cop[j][k + 1] += dp[j][k] * (modint)x;
				//put on horyu
				if (k > 0) {
					cop[j][k - 1] += dp[j][k];
				}
				//to horyu
				cop[j + 1][k] += dp[j][k];
			}

			rep(j, n + 1)rep(k, n + 1)dp[j][k] = cop[j][k];
		}
	}
	modint ans = dp[0][0];
	cout << ans << "\n";
}




signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	//cout << fixed << setprecision(10);
	//init();
	//int t; cin >> t; rep(i, t)
	solve();
	return 0;
}


Measured memory_bytes: 21468