#include <bits/stdc++.h>

using namespace std;

using int64 = long long;
const int mod = 1e9 + 7;

const int64 infll = (1LL << 58) - 1;
const int inf = (1 << 30) - 1;

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


template< typename T1, typename T2 >
ostream &operator<<(ostream &os, const pair< T1, T2 > &p) {
  os << p.first << " " << p.second;
  return os;
}

template< typename T1, typename T2 >
istream &operator>>(istream &is, pair< T1, T2 > &p) {
  is >> p.first >> p.second;
  return is;
}

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< typename F >
struct FixPoint : F {
  FixPoint(F &&f) : F(forward< F >(f)) {}

  template< typename... Args >
  decltype(auto) operator()(Args &&... args) const {
    return F::operator()(*this, forward< Args >(args)...);
  }
};

template< typename F >
inline decltype(auto) MFP(F &&f) {
  return FixPoint< F >{forward< F >(f)};
}

template< int mod >
struct ModInt {
  int x;

  ModInt() : x(0) {}

  ModInt(int64_t 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;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }

  ModInt pow(int64_t n) const {
    ModInt ret(1), mul(x);
    while(n > 0) {
      if(n & 1) ret *= mul;
      mul *= mul;
      n >>= 1;
    }
    return ret;
  }

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

  friend istream &operator>>(istream &is, ModInt &a) {
    int64_t t;
    is >> t;
    a = ModInt< mod >(t);
    return (is);
  }

  static int get_mod() { return mod; }
};

using modint = ModInt< mod >;

int dp[2][3001][3001];
int nxt_dp[2][3001][3001];

int main() {
  int N;
  cin >> N;
  vector< int > low(N + 1), high(N + 1);
  for(int i = 0; i < N; i++) {
    int t, x;
    cin >> t >> x;
    if(t == 0) {
      low[x]++;
    } else {
      high[x - 1]++;
    }
  }
  for(int i = N - 1; i >= 0; i--) {
    low[i] += low[i + 1];
    high[i] += high[i + 1];
  }


  dp[0][0][0] = 1;
  int x_lim = 1, y_lim = 1;

  for(int i = N; i > 0; i--) {
    for(int l = 0; l < 2; l++) {
      for(int j = 0; j < x_lim; j++) {
        for(int m = 0; m < y_lim; m++) {
          nxt_dp[l][j][m] = dp[l][j][m];
        }
      }
    }
    for(int l = 0; l < 2; l++) {
      for(int j = 0; j < low[i] && j < x_lim; j++) {
        for(int k = 0; k < y_lim; k++) {
          nxt_dp[l][j + 1][k] += dp[l][j][k] * (low[i] - j);
        }
      }
    }
    for(int l = 0; l < 2; l++) {
      for(int j = 0; j < x_lim; j++) {
        for(int k = 0; k < high[i] && k < y_lim; k++) {
          nxt_dp[l ^ 1][j][k + 1] += dp[l][j][k] * (high[i] - k);
        }
      }
    }
    x_lim = min(x_lim + 1, low[0] + 1);
    y_lim = min(y_lim + 1, high[0] + 1);
    for(int l = 0; l < 2; l++) {
      for(int j = 0; j < x_lim; j++) {
        for(int k = 0; k < x_lim; k++) {
          dp[l][j][k] = nxt_dp[l][j][k];
        }
      }
    }
  }
  vector< modint > muls(N + 1);
  muls[0] = 1;
  for(int k = 1; k <= N; k++) muls[k] = muls[k - 1] * k;

  modint add = 0;
  for(int k = 0; k <= high[0]; k++) {
    int rest = N - low[0] - k;
    if(rest >= 0) {
      modint p = dp[0][low[0]][k] - dp[1][low[0]][k];
      p *= muls[rest];
      add += p;
    }
  }
  cout << add << endl;
}