#include <bits/stdc++.h>
#define MP make_pair
#define PB push_back
#define int long long
#define st first
#define nd second
#define rd third
#define FOR(i, a, b) for(int i =(a); i <=(b); ++i)
#define RE(i, n) FOR(i, 1, n)
#define FORD(i, a, b) for(int i = (a); i >= (b); --i)
#define REP(i, n) for(int i = 0;i <(n); ++i)
#define VAR(v, i) __typeof(i) v=(i)
#define FORE(i, c) for(VAR(i, (c).begin()); i != (c).end(); ++i)
#define ALL(x) (x).begin(), (x).end()
#define SZ(x) ((int)(x).size())
#define __builtin_ctz __builtin_ctzll
#define __builtin_clz __builtin_clzll
#define __builtin_popcount __builtin_popcountll
using namespace std;
template<typename TH> void _dbg(const char* sdbg, TH h) { cerr<<sdbg<<"="<<h<<"\n"; }
template<typename TH, typename... TA> void _dbg(const char* sdbg, TH h, TA... t) {
  while(*sdbg != ',') { cerr<<*sdbg++; } cerr<<"="<<h<<","; _dbg(sdbg+1, t...);
}
#ifdef LOCAL
#define debug(...) _dbg(#__VA_ARGS__, __VA_ARGS__)
#define debugv(x) {{cerr <<#x <<" = "; FORE(itt, (x)) cerr <<*itt <<", "; cerr <<"\n"; }}
#else
#define debug(...) (__VA_ARGS__)
#define debugv(x)
#define cerr if(0)cout
#endif
#define next ____next
#define prev ____prev
#define left ____left
#define hash ____hash
typedef long long ll;
typedef long double LD;
typedef pair<int, int> PII;
typedef pair<ll, ll> PLL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<ll> VLL;
typedef vector<pair<int, int> > VPII;
typedef vector<pair<ll, ll> > VPLL;

template<class C> void mini(C&a4, C b4){a4=min(a4, b4); }
template<class C> void maxi(C&a4, C b4){a4=max(a4, b4); }
template<class T1, class T2>
ostream& operator<< (ostream &out, pair<T1, T2> pair) { return out << "(" << pair.first << ", " << pair.second << ")";}
template<class A, class B, class C> struct Triple { A first; B second; C third;
  bool operator<(const Triple& t) const { if (st != t.st) return st < t.st; if (nd != t.nd) return nd < t.nd; return rd < t.rd; } };
template<class T> void ResizeVec(T&, vector<int>) {}
template<class T> void ResizeVec(vector<T>& vec, vector<int> sz) {
  vec.resize(sz[0]); sz.erase(sz.begin()); if (sz.empty()) { return; }
  for (T& v : vec) { ResizeVec(v, sz); }
}
typedef Triple<int, int, int> TIII;
template<class A, class B, class C>
ostream& operator<< (ostream &out, Triple<A, B, C> t) { return out << "(" << t.st << ", " << t.nd << ", " << t.rd << ")"; }
template<class T> ostream& operator<<(ostream& out, vector<T> vec) { out<<"("; for (auto& v: vec) out<<v<<", "; return out<<")"; }
template<class T> ostream& operator<<(ostream& out, set<T> vec) { out<<"("; for (auto& v: vec) out<<v<<", "; return out<<")"; }
template<class L, class R> ostream& operator<<(ostream& out, map<L, R> vec) { out<<"("; for (auto& v: vec) out<<v<<", "; return out<<")"; }


// ide od
const int N = 610;
int dp[N][N / 2][N / 2]; // dp[to_process][need_smaller_indistin][need_smaller_distin]
int used[N];
int a[N];
int single_at_most[N];
const int P = 1e9 + 7;
void AddToSelf(int& a, int b) {
  if (b >= P) { b %= P; }
  a += b;
  if (a >= P) {
    a -= P;
  }
}
int32_t main() {

  ios_base::sync_with_stdio(0);
  cout << fixed << setprecision(10);
  cerr << fixed << setprecision(10);
  cin.tie(0);
  //double beg_clock = 1.0 * clock() / CLOCKS_PER_SEC;
  
  int n;
  cin>>n;
  RE (i, 2 * n) {
    cin>>a[i];
  }
  VI singles;
  int both = 0;
  RE (i, n) {
    if (a[2 * i - 1] != -1 && a[2 * i] != -1) {
      used[a[2 * i - 1]] = 2;
      used[a[2 * i]] = 2;
    } else if (a[2 * i - 1] == -1 && a[2 * i] == -1) {
      both++;
    } else {
      used[a[2 * i - 1] + a[2 * i] + 1] = 1;
      single_at_most[a[2 * i - 1] + a[2 * i] + 1] = 1;
    }
  }
  RE (i, 2 * n) {
    single_at_most[i] += single_at_most[i - 1];
  }
  dp[2 * n][0][0] = 1;
  FORD (i, 2 * n, 1) {
    if (used[i] == 2) {
      REP (a, n + 1) {
        REP (b, n + 1) {
          dp[i - 1][a][b] = dp[i][a][b];
        }
      }
    } else {
      if (used[i] == 1) {
        REP (prv_indistin, n + 1) {
          REP (prv_distin, n + 1) {
            if (prv_indistin) {
              AddToSelf(dp[i - 1][prv_indistin - 1][prv_distin], dp[i][prv_indistin][prv_distin]);
            }
            AddToSelf(dp[i - 1][prv_indistin][prv_distin + 1], dp[i][prv_indistin][prv_distin]);
          }
        }
      } else {
        REP (prv_indistin, n + 1) {
          REP (prv_distin, n + 1) {
            if (prv_distin) {
              AddToSelf(dp[i - 1][prv_indistin][prv_distin - 1], prv_distin * dp[i][prv_indistin][prv_distin]);
            }
            if (prv_indistin) {
              AddToSelf(dp[i - 1][prv_indistin - 1][prv_distin], dp[i][prv_indistin][prv_distin]);
            }
            AddToSelf(dp[i - 1][prv_indistin + 1][prv_distin], dp[i][prv_indistin][prv_distin]);
          }
        }
      } 
    }
  }
  int res = dp[0][0][0];
  RE (i, both) {
    res = res * i % P;
  }
  cout<<res<<endl;
  return 0;
}


Measured memory_bytes: 436480