#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <climits>
#include <cmath>
#include <complex>
#include <cstring>
#include <deque>
#include <functional>
#include <iostream>
#include <iomanip>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <unordered_map>
#include <unordered_set>
#include <vector>
#include <cstdint>
using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> pii;
#define MP make_pair
#define PB push_back
#define inf 1000000007
#define rep(i,n) for(int i = 0; i < (int)(n); ++i)
#define all(x) (x).begin(),(x).end()

template<typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val){
    std::fill( (T*)array, (T*)(array+N), val );
}
 
template<class T> inline bool chmax(T &a, T b){
    if(a<b){
        a = b;
        return true;
    }
    return false;
}

template<class T> inline bool chmin(T &a, T b){
    if(a>b){
        a = b;
        return true;
    }
    return false;
}


template <unsigned int mod>
class ModInt {
private:
    unsigned int v;
    static unsigned int norm(const unsigned int& x){ return x < mod ? x : x - mod; }
    static ModInt make(const unsigned int& x){ ModInt m; return m.v = x, m; }
    static ModInt inv(const ModInt& x){ return make(inverse(x.v, mod)); }
    static unsigned int inverse(int a, int m){
        int u[] = {a, 1, 0}, v[] = {m, 0, 1}, t;
        while(*v){
            t = *u / *v;
            swap(u[0] -= t * v[0], v[0]), swap(u[1] -= t * v[1], v[1]), swap(u[2] -= t * v[2], v[2]);
        }
        return (u[1] % m + m) % m;
    }

public:
    ModInt() : v{0}{}
    ModInt(const long long val) : v{norm(val % mod + mod)} {}
    ModInt(const ModInt<mod>& n) : v{n()} {}
    explicit operator bool() const noexcept { return v != 0; }
    bool operator!() const noexcept { return !static_cast<bool>(*this); }
    ModInt& operator=(const ModInt& n){ return v = n(), (*this); }
    ModInt& operator=(const long long val){ return v = norm(val % mod + mod), (*this); }
    ModInt operator+() const { return *this; }
    ModInt operator-() const { return v == 0 ? make(0) : make(mod - v); }
    ModInt operator+(const ModInt& val) const { return make(norm(v + val())); }
    ModInt operator-(const ModInt& val) const { return make(norm(v + mod - val())); }
    ModInt operator*(const ModInt& val) const { return make((long long)v * val() % mod); }
    ModInt operator/(const ModInt& val) const { return *this * inv(val); }
    ModInt& operator+=(const ModInt& val){ return *this = *this + val; }
    ModInt& operator-=(const ModInt& val){ return *this = *this - val; }
    ModInt& operator*=(const ModInt& val){ return *this = *this * val; }
    ModInt& operator/=(const ModInt& val){ return *this = *this / val; }
    ModInt operator+(const long long val) const { return ModInt{v + val}; }
    ModInt operator-(const long long val) const { return ModInt{v - val}; }
    ModInt operator*(const long long val) const { return ModInt{(long long)v * (val % mod)}; }
    ModInt operator/(const long long val) const { return ModInt{(long long)v * inv(val)}; }
    ModInt& operator+=(const long long val){ return *this = *this + val; }
    ModInt& operator-=(const long long val){ return *this = *this - val; }
    ModInt& operator*=(const long long val){ return *this = *this * val; }
    ModInt& operator/=(const long long val){ return *this = *this / val; }
    bool operator==(const ModInt& val) const { return v == val.v; }
    bool operator!=(const ModInt& val) const { return !(*this == val); }
    bool operator==(const long long val) const { return v == norm(val % mod + mod); }
    bool operator!=(const long long val) const { return !(*this == val); }
    unsigned int operator()() const { return v; }
    friend ModInt operator+(const long long val, const ModInt& n) { return n + val; }
    friend ModInt operator-(const long long val, const ModInt& n) { return ModInt{val - n()}; }
    friend ModInt operator*(const long long val, const ModInt& n) { return n * val; }
    friend ModInt operator/(const long long val, const ModInt& n) { return ModInt{val} / n; }
    friend bool operator==(const long long val, const ModInt& n) { return n == val; }
    friend bool operator!=(const long long val, const ModInt& n) { return !(val == n); }
    friend istream& operator>>(istream& is, ModInt& n){
        unsigned int v;
        return is >> v, n = v, is;
    }
    friend ostream& operator<<(ostream& os, const ModInt& n){ return (os << n()); }
    friend ModInt mod_pow(ModInt x, long long n){
        ModInt ans = 1;
        while(n){
            if(n & 1) ans *= x;
            x *= x, n >>= 1;
        }
        return ans;
    }
};

#define MOD 1000000007
using mod = ModInt<MOD>;

#define MAX_N 200000
mod inv[MAX_N],fac[MAX_N],finv[MAX_N];

void make()
{
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for(int i=2;i<MAX_N;i++){
        inv[i] = MOD - inv[MOD % i] * (MOD / i);
        fac[i] = fac[i-1] * i;
        finv[i] = finv[i-1] * inv[i];
    }
}

mod comb(int a, int b)
{
    if(a<b) return 0;
    return fac[a] * finv[b] * finv[a-b];
}

mod dp[603][603][603];
int cc[603];
int main(){
    make();
    int n;
    cin >> n;
    vector<int> a(2*n);
    rep(i,2*n) cin >> a[i];
    vector<bool> p(2*n+1);
    p[0] = 1;
    vector<int> b;
    rep(i,n){
        if(a[2*i]!=-1&&a[2*i+1]!=-1){
            p[a[2*i]] = 1;
            p[a[2*i+1]] = 1;
        }else{
            b.push_back(a[2*i]);
            b.push_back(a[2*i+1]);
        }
    }
    map<int,int> mp;
    mp[-1] = -1;
    int c = 1;
    rep(i,2*n+1){
        if(!p[i]){
            mp[i] = c;
            c++;
        }
    }
    int m = b.size();
    m /= 2;
    rep(i,m){
        b[2*i] = mp[b[2*i]];
        b[2*i+1] = mp[b[2*i+1]];
    }
    set<int> st;
    int T = 0;
    rep(i,m){
        cerr << b[2*i] << " " << b[2*i+1] << endl;
        if(b[2*i]==-1&&b[2*i+1]==-1){
            T++;
        }else{
            st.insert(max(b[2*i],b[2*i+1]));
            cc[max(b[2*i],b[2*i+1])]++;
        }
    }
    for(int i=0;i<2*m;i++){
        cc[i+1] += cc[i];
    }
    dp[2*m][0][T] = 1;
    for(int k=2*m;k>=1;k--){
        for(int s=0;s<=2*m;s++){
            for(int t=0;t<=T;t++){
                int u = s+2*t+2*cc[k] - k;
                if(u<0)continue;
                if(st.count(k)){
                    dp[k-1][s+1][t] += dp[k][s][t];
                    dp[k-1][s][t] += dp[k][s][t];
                }else{
                    dp[k-1][s][t] += dp[k][s][t];
                    if(t!=0) dp[k-1][s][t-1] += dp[k][s][t];
                    // if(t!=0) dp[k-1][s+1][t-1] += dp[k][s][t];
                    if(s!=0) dp[k-1][s-1][t] += dp[k][s][t]*s;
                }
            }
        }
    }
    cout << dp[0][0][0]*fac[T] << endl;
    return 0;
}

Measured memory_bytes: 859640