/*
                                                                                                     
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                                         .o:--..`` ``..--:o`                                        
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                                       `/y+o/---....---:+o.                                         
                                   `...````-os+/:---:/+o/--.`                                       
              `-/+++++/:.      `...`       :h+d+oooo+/+-`   ...                                     
            `/++//:::://++-`....`         -.`//````````:`     `..`                                  
           `o+/::------://o/`           `-` -.          -`       `..`                               
 `---.-o/:./o/::-..``..-ЗАПУСКАЕМ      ..  ..            -`        `...       ``..``                
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          `:s+/:--....-::/+s-`      .-   `-                -`           -///:--------:/:`           
           ./s+//:::::://oo-``..НЕЙРОННУЮ: СЕТЬ:::::::-`РАБОТЯГИ        `+:--........--:/`          
            .:ooo+++++osso-`    `.:-...`/` ./::-------:/:`   -`         :+--..``````.--:+:...-+:-`  
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             `:++++/+++++-  ..``.-/:`      /y-:/++o++/:.`..`       ./.   `-                         
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            `./+oooooooo+/.`-    .-:...`.. .//:-------://`        `- `..` `:.                       
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                       ..`-.   `-.   ``:os:o/:---...---:++.  `-     ``///+:-..``````.--:+-````-.`   
              `.:///////.-`      .:-..` -``-+o+/:::::/+o/.  `-         `:+:-..`````..--:o/:--/ys+-  
            `-++///////+o/. ``....`-.    :` `.:++++++/:.`  .-           -o/---......---/o.   `.`    
           `++//:-----::/+o:..`     .-`   :    ```````    .-           `+so+:--------:++-`          
  `````:-``:o/::-..`..--:/+o`         -.  `-             .-          `../../+o+////+o+:.`           
  -----syo/o+/:--.```..-://s.          .-` `-           .-        `...     ``-:////:-``             
       .` `/s//:--....-::/+s.            -. `-`        .-       `..`                                
           .+o+/:::--:://+s/-..`          .::+y  ```  .-     `..`                                   
            ./oo++////+oso-`   `....       :y-+:::::::/`   ...                                      
             `.:+oooooo/-`         `....-. .//:-------:/:-.`                                        
                ``...``                 /+:+:--.......--:+`                                         
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                                         .o:--..`` ``..--:o`                                        
                                         .+/--...```..--:+/`                                        
                                         `-o/:---...---:++.                                         
                                          `-+o+/:---:/+o/.                                          
                                            `.:+oooo+/-.`                                           
                                               ``````                                               
*/

#ifdef aimbot
#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#pragma GCC optimize("unroll-loops")
#endif

#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <map>
#include <set>
#include <queue>
#include <ostream>
#include <istream>
#include <typeinfo>
#include <iomanip>
#include <cstdio>
#include <cstdlib>
#include <cassert>
#include <limits>
#include <fstream>
#include <array>
#include <list>
#include <bitset>
#include <functional>
#include <random>
#include <cstring>
#include <chrono>

#define random escape__from__random__aetuhoetnuhshe
#define mt make_tuple
#define x first
#define y second
#define pb push_back
#define ppb pop_back
#define mp make_pair
#define umap unordered_map
#define uset unordered_set
#define elif else if
#define len(v) ((int)v.size())
#define f(i, n) for (int i = 0; i < (n); i++)
#define rof(i, n) for (int i = ((n) - 1); i >= 0; i--)
#define apply(v, act) for (auto &x : v) { act; }
#define log(args...) {string s = #args;deque<string> deq;\
string buf = "";int bal = 0;for (char c : s) {\
if (c == '(' || c == '[' || c == '{') {bal++;\
} else if (c == ')' || c == ']' || c == '}') {\
bal--;} else {if (bal == 0) {if (c == ',') {\
deq.pb(buf);buf = "";} else {if (c != ' ') {\
buf += c;}}}}}if (!buf.empty()) {deq.pb(buf);}\
smart_io::precall_print();smart_io::_print(deq, args);}

inline int min(const int &x, const int &y) { return (((y-x)>>(32-1))&(x^y))^x; }
inline int max(const int &x, const int &y) { return (((y-x)>>(32-1))&(x^y))^y; }
inline long long min(const long long &x, const long long &y) { return (((y-x)>>(64-1))&(x^y))^x; }
inline long long max(const long long &x, const long long &y) { return (((y-x)>>(64-1))&(x^y))^y; }

#define print    \
smart_io::precall_print(); \
cout,

#define scan cin,

#ifdef fast_allocator
const int MAXMEM = 200 * 1000 * 1024;
char _memory[MAXMEM];
size_t _ptr = 0;
void* operator new(size_t _x) { _ptr += _x; assert(_ptr < MAXMEM); return _memory + _ptr - _x; }
void operator delete (void*) noexcept {}
#endif

using namespace std;

char string_in_buffer[(int)260];


void fast_scan(int &x) { scanf("%d", &x); }
void fast_scan(long long &x) { scanf("%lld", &x); }
void fast_scan(unsigned long long &x) { scanf("%llu", &x); }
void fast_scan(double &x) { scanf("%lf", &x); }
void fast_scan(long double &x) { scanf("%Lf", &x); }
void fast_scan(char &x) {
	scanf("%c", &x);
	if (x == '\n') {
		fast_scan(x);
	}
}
void fast_scan(string &x) {
	scanf("%s", string_in_buffer);
	x = string(string_in_buffer);
}

template<class TFirst, class TSecond>
void fast_scan(pair<TFirst, TSecond> &p) {
	fast_scan(p.first);
	fast_scan(p.second);
}

template <class T>
void fast_scan(vector<T> &v) {
	for (auto &x : v) fast_scan(x);
}

void fast_print(const int &x) { printf("%d", x); }
void fast_print(const unsigned int &x) { printf("%u", x); }
void fast_print(const long long &x) { printf("%lld", x); }
void fast_print(const unsigned long long &x) { printf("%llu", x); }
void fast_print(const char &x) { printf("%c", x); };
void fast_print(__int128 x) {
	if (x == 0) { fast_print('0'); return; }
	if (x < 0) {
		fast_print('-');
		x = -x;
	}
	__int128 p = 1;
	while (x / (p * 10)) p *= 10;
	while (p) {
		__int128 symb = x / p;
		fast_print((int)symb);
		x -= p * symb;
		p /= 10;
	}
};
void fast_print(const double &x) { printf("%.15lf", x); }
void fast_print(const long double &x) { printf("%.15Lf", x); }
void fast_print(const string &x) { printf("%s", x.c_str());}
void fast_print(const char v[]) { fast_print((string)v); }

template<class TFirst, class TSecond>
void fast_print(const pair<TFirst, TSecond> &p) {
	fast_print(p.first);
	fast_print(' ');
	fast_print(p.second);
}

template <class T>
void fast_print(const vector<T> &v) {
	if (v.empty()) return;
	fast_print(v[0]);
	for (int i = 1; i < v.size(); i++) {
		fast_print(' ');
		fast_print(v[i]);
	}
}

template <class T>
void fast_print(const vector<vector<T>> &v) {
	if (v.empty()) return;
	fast_print(v[0]);
	for (int i = 1; i < v.size(); i++) {
		fast_print('\n');
		fast_print(v[i]);
	}
}

template <class T>
void fast_print(const T &v) {
	for (const auto &x : v) {
		fast_print(x);
		fast_print(' ');
	}
}


using namespace std;


namespace smart_io {
	string print_start = "";
	string sep = " ";
	bool first_print = false;
	
	void precall_print() {
		fast_print(print_start);
		print_start = "\n";
		first_print = true;
	}
	
	void _print(deque<string>) {}
	template<class T, class... Args>
	void _print(deque<string> names, T elem, Args... args) {
		if (!first_print) {
			fast_print("\n");
		} else {
			first_print = false;
		}
		fast_print(names.front());
		fast_print(" = ");
		fast_print(elem);
		names.pop_front();
		_print(names, args...);
	}
} //namespace smart_io


template <class T>
ostream &operator,(ostream &os, const T &object) {
	if (!smart_io::first_print) {
		fast_print(smart_io::sep);
	} else {
		smart_io::first_print = false;
	}
	fast_print(object);
	return os;
}

template <class T>
istream &operator,(istream &is, T &object) {
	fast_scan(object);
	return is;
}

namespace random {
	using namespace std::chrono;
	mt19937 rng(duration_cast< milliseconds >(
											  system_clock::now().time_since_epoch()
											  ).count());
	uniform_real_distribution<> prob_dist(0.0, 1.0);
};
#define int long long
namespace typedefs {
	typedef long long ll;
	typedef unsigned long long ull;
	typedef pair<int, int> pii;
	typedef long double ld;
}

namespace numbers_operation {
	template<class T>
	T floor_mod(T a, const T &b) {
		a %= b;	
		if (a < 0) a += b;
		return a;
	}
}

using namespace numbers_operation;
using namespace typedefs;
using namespace random;

const ll MOD = 1e9 + 7;

int n;
vector<int> a;

int dp[700][700], ndp[700][700];

void add(ll &x, ll y) {
	x += y;
	x -= MOD * (x >= MOD);
}

signed main(signed argc, char *argv[]) {
	scan n;	
	n *= 2;
	a.resize(n);
	scan a;
	apply(a, x = ((x == -1) ? -1 : x - 1));
	int free = 0;
	vector<pii> v;
	for (int i = 0; i < n; i += 2) {
		if (a[i] == -1 && a[i + 1] == -1) {
			free++;
		} else if ((a[i] == -1) || (a[i + 1] == -1)) {
			int value = a[i] + a[i + 1] + 1;
			v.emplace_back(value, 0);
		}
	}
	for (int i = 0; i < n; i++) {
		if (find(a.begin(), a.end(), i) == a.end()) {
			v.emplace_back(i, 1);
		}
	}
	sort(v.begin(), v.end());
	// print v;
	// print free;
	// int dp[2 * n + 1][2 * n + 1]; // official && not official 
	// f(i, 2 * n + 1)
	// 	fill(dp[i], dp[i] + 2 * n + 1, 0);
	int sum_off = 0, sum_unn = 0;
	dp[0][0] = 1;
	for (int i = len(v) - 1; i > -1; i--) {
		// int ndp[2 * n + 1][2 * n + 1];
		// f(i, 2 * n + 1)
			// fill(ndp[i], ndp[i] + 2 * n + 1, 0);
		// for (int a = 0; a < n + 1; a++) {
		// 	print vector<int>(dp[a], dp[a] + n + 1);
		// }
		// print "__", v[i].y;

		memset(ndp, 0, sizeof ndp);
		if (v[i].y == 0) { // official
			// combine with unnofficial
			for (int a = 0; a < n + 1; a++)
				for (int b = 1; b < n + 1; b++)
					add(ndp[a][b - 1], dp[a][b]);
			// add to wait queue
			for (int a = 0; a < n; a++)
				for (int b = 0; b < n + 1; b++)
					add(ndp[a + 1][b], dp[a][b]);
			sum_off++;
		} else { // not official
			// combine with official
			for (int a = 1; a < n + 1; a++)
				for (int b = 0; b < n + 1; b++)
					add(ndp[a - 1][b], (dp[a][b] * a) % MOD);
			// add to wait queue
			for (int a = 0; a < n + 1; a++) 
				for (int b = 0; b < n; b++)
					add(ndp[a][b + 1], dp[a][b]);
			// combine with unnofficial?? -> exactly 
			for (int a = 0; a < n + 1; a++)
				for (int b = 1; b < n + 1; b++) {
					int cur = a - b;
					if (floor_mod(cur, 2LL) == floor_mod(sum_off - sum_unn, 2LL)) {
						int remain = free - abs(cur - (sum_off - sum_unn)) / 2;
						if (remain > 0) {
							add(ndp[a][b - 1], (dp[a][b] * remain) % MOD);
						}
					}
				}
			sum_unn++;
		}
		swap(dp, ndp);
	}
	print dp[0][0];
}


Measured memory_bytes: 8064