#include<bits/stdc++.h>

using namespace std;

using int64 = long long;
const int mod = 1e9 + 7;
const int inf = (1 << 30) - 1;
const int64 infll = (1LL << 61) - 1;

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;

template< typename T >
ostream &operator<<(ostream &os, const vector< T > &v) {
  for(int i = 0; i < (int) v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

template< typename T >
istream &operator>>(istream &is, vector< T > &v) {
  for(T &in : v) is >> in;
  return is;
}

template< typename T1, typename T2 >
inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }

template< typename T1, typename T2 >
inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }

template< typename T = int64 >
vector< T > make_v(size_t a) {
  return vector< T >(a);
}

template< typename T, typename... Ts >
auto make_v(size_t a, Ts... ts) {
  return vector< decltype(make_v< T >(ts...)) >(a, make_v< T >(ts...));
}

template< typename T, typename V >
typename enable_if< is_class< T >::value == 0 >::type fill_v(T &t, const V &v) {
  t = v;
}

template< typename T, typename V >
typename enable_if< is_class< T >::value != 0 >::type fill_v(T &t, const V &v) {
  for(auto &e : t) fill_v(e, v);
}

template< int mod >
struct ModInt {
  int x;

  ModInt() : x(0) {}

  ModInt(long long y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

  ModInt &operator+=(const ModInt &p) {
    if((x += p.x) >= mod) x -= mod;
    return *this;
  }

  ModInt &operator-=(const ModInt &p) {
    if((x += mod - p.x) >= mod) x -= mod;
    return *this;
  }

  ModInt &operator*=(const ModInt &p) {
    x = (int) (1LL * x * p.x % mod);
    return *this;
  }

  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }

  ModInt operator-() const { return ModInt(-x); }

  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }

  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }

  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }

  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

  bool operator==(const ModInt &p) const { return x == p.x; }

  bool operator!=(const ModInt &p) const { return x != p.x; }

  ModInt inverse() const {
    int a = x, b = mod, u = 1, v = 0, t;
    while(b > 0) {
      t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    return ModInt(u);
  }

  friend ostream &operator<<(ostream &os, const ModInt< mod > &p) {
    return os << p.x;
  }

  friend istream &operator>>(istream &is, ModInt< mod > &a) {
    long long x;
    is >> x;
    a = ModInt< mod >(x);
    return (is);
  }

};

using modint = ModInt< mod >;


modint dp[602][602][602];
int type[602]; // -1: 決定済み, 1: 片方のみ

int main() {
  int N;
  cin >> N;

  int undef = 0;
  for(int i = 0; i < N; i++) {
    int a, b;
    cin >> a >> b;
    if(a > b) swap(a, b);

    if(b == -1) {
      ++undef;
    } else if(a == -1) {
      type[b] = 1;
    } else {
      type[a] = type[b] = -1;
    }
  }


  dp[2 * N][0][0] = 1;
  for(int i = 1; i <= undef; i++) dp[2 * N][0][0] *= i;

  for(int i = 2 * N; i >= 1; i--) {
    for(int a = 0; a <= N; a++) {
      for(int b = 0; b <= N; b++) {
        if(dp[i][a][b] == 0) continue;

        if(type[i] == 0) {
          dp[i - 1][a + 1][b] += dp[i][a][b];
          if(a) dp[i - 1][a - 1][b] += dp[i][a][b];
          if(b) dp[i - 1][a][b - 1] += dp[i][a][b] * b;
        } else if(type[i] == 1) {
          dp[i - 1][a][b + 1] += dp[i][a][b];
          if(a) dp[i - 1][a - 1][b] += dp[i][a][b];
        } else {
          dp[i - 1][a][b] += dp[i][a][b];
        }
      }
    }
  }
  cout << dp[0][0][0] << endl;
}









Measured memory_bytes: 852608