#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define rep1(i,n) for(int i=1;i<=(int)(n);i++)
#define all(c) c.begin(),c.end()
#define pb push_back
#define fs first
#define sc second
#define show(x) cout << #x << " = " << (x) << endl
#define chmin(x,y) x=min(x,y)
#define chmax(x,y) x=max(x,y)
using namespace std;
template<class S,class T> ostream& operator<<(ostream& o,const pair<S,T> &p){
	return o<<"("<<p.fs<<","<<p.sc<<")";
}
template<class T> ostream& operator<<(ostream& o,const vector<T> &vc){
	o<<"{";
	for(const T& v:vc) o<<v<<",";
	o<<"}";
	return o;
}
using ll = long long;
template<class T> using V = vector<T>;
template<class T> using VV = vector<vector<T>>;
template<unsigned int mod_>
struct ModInt{
	using uint = unsigned int;
	using ll = long long;
	using ull = unsigned long long;
 
	constexpr static uint mod = mod_;
 
	uint v;
	ModInt():v(0){}
	ModInt(ll v):v(normS(v%mod+mod)){}
	explicit operator bool() const {return v!=0;}
	static uint normS(const uint &x){return (x<mod)?x:x-mod;}		// [0 , 2*mod-1] -> [0 , mod-1]
	static ModInt make(const uint &x){ModInt m; m.v=x; return m;}
	ModInt operator+(const ModInt& b) const { return make(normS(v+b.v));}
	ModInt operator-(const ModInt& b) const { return make(normS(v+mod-b.v));}
	ModInt operator-() const { return make(normS(mod-v)); }
	ModInt operator*(const ModInt& b) const { return make((ull)v*b.v%mod);}
	ModInt operator/(const ModInt& b) const { return *this*b.inv();}
	ModInt& operator+=(const ModInt& b){ return *this=*this+b;}
	ModInt& operator-=(const ModInt& b){ return *this=*this-b;}
	ModInt& operator*=(const ModInt& b){ return *this=*this*b;}
	ModInt& operator/=(const ModInt& b){ return *this=*this/b;}
	ll extgcd(ll a,ll b,ll &x,ll &y) const{
		ll u[]={a,1,0},v[]={b,0,1};
		while(*v){
			ll t=*u/ *v;
			rep(i,3) swap(u[i]-=t*v[i],v[i]);
		}
		if(u[0]<0) rep(i,3) u[i]=-u[i];
		x=u[1],y=u[2];
		return u[0];
	}
	ModInt inv() const{
		ll x,y;
		extgcd(v,mod,x,y);
		return make(normS(x+mod));
	}
	bool operator==(const ModInt& b) const { return v==b.v;}
	bool operator!=(const ModInt& b) const { return v!=b.v;}
	friend istream& operator>>(istream &o,ModInt& x){
		ll tmp;
		o>>tmp;
		x=ModInt(tmp);
		return o;
	}
	friend ostream& operator<<(ostream &o,const ModInt& x){ return o<<x.v;}
};
using mint = ModInt<1000000007>;
 
mint solve(int N,V<int> one){
//	show(N);
//	show(one);
	V<bool> isone(N+N);
	for(int x:one) isone[x] = true;
 
	const int X = 610;
	VV<mint> dp(X,V<mint>(X));
	VV<mint> ndp = dp;
	dp[0][0] = 1;
 
	for(int i=N+N-1;i>=0;i--){
//		cerr << " ---------------- i = " << i << " --------------" << endl;
		rep(j,X) rep(k,X) ndp[j][k] = 0;
 
		if(isone[i]){
			rep(w,X) rep(c,X) if(dp[w][c]){
				if(w) ndp[w-1][c] += dp[w][c];
				ndp[w][c+1] += dp[w][c];
			}
		}else{
			rep(w,X) rep(c,X) if(dp[w][c]){
				if(w) ndp[w-1][c] += dp[w][c];
				if(c) ndp[w][c-1] += dp[w][c] * c;
				ndp[w+1][c] += dp[w][c];
			}
		}
 
		dp = ndp;
	}
	mint res = dp[0][0];
	int F = N - (int)one.size();
	rep1(i,F) res *= i;
	return res;
}
 
int main(){
	int N;
	cin>>N;
	V<int> a(N+N);
	rep(i,N+N){
		cin>>a[i];
		if(a[i]>=1) a[i]--;
	}
	V<bool> er(N+N);
	rep(i,N){
		if(a[i*2] != -1 && a[i*2+1] != -1){
			er[a[i*2]] = er[a[i*2+1]] = true;
		}
	}
	V<int> lefts;
	rep(i,N+N) if(!er[i]) lefts.pb(i);
	int nN = lefts.size()/2;
	V<int> one;
	rep(i,N+N) if(a[i] != -1 && a[i^1] == -1){
		one.pb(a[i]);
	}
	for(int& x: one) x = lower_bound(all(lefts),x) - lefts.begin();
	cout<<solve(nN,one)<<endl;
}

Measured memory_bytes: 3200