#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 C[1000][1000];
void init(){
	rep(i,1000){
		C[i][0] = C[i][i] = 1;
		rep1(j,i-1) C[i][j] = C[i-1][j-1] + C[i-1][j];
	}
}

mint Choose(int a,int b){
	if(b<0 || b>a) return 0;
	return C[a][b];
}

mint solve(int N,V<int> one){
//	show(N);
//	show(one);
	V<bool> isone(N+N);
	for(int x:one) isone[x] = true;

	VV<mint> dp(N+2,V<mint>(N+2));
	VV<mint> ndp = dp;
	dp[0][0] = 1;
	rep(i,N+N){
//		cerr << " ---------------- i = " << i << " --------------" << endl;
		int L = 0;
		for(int j=i;j<N+N;j++) if(isone[j]) L++;
		rep(j,N+1) rep(k,N+1) ndp[j][k] = 0;
		rep(g,N+1) rep(f,g+1) if(dp[g][f]){
//			printf("dp[%d][%d] = %d\n",g,f,dp[g][f].v);
			mint val = dp[g][f];
			if(isone[i]){
				int freecolornum = N - (i+g)/2;
				int freecolorL = L - f;
				int freefree = freecolornum - freecolorL;

				mint valblack = val * Choose(L-1,f-1) / Choose(L,f);
				mint valwhite = val - valblack;
				// show(L);
				// show(f);
				// show(valblack);
				// show(valwhite);
				if(valblack){	//black(fixed)
					if(f && g) ndp[g-1][f-1] += valblack;
				}
				if(valwhite){
					ndp[g+1][f] += valwhite;
				}
			}else{
				int freecolornum = N - (i+g)/2;
				int freecolorL = L - f;
				int freefree = freecolornum - freecolorL;
				ndp[g+1][f] += val * freefree;
				ndp[g+1][f+1] += val * freecolorL;
				if(g) ndp[g-1][f] += val;
			}
		}
		rep(j,N+1) rep(k,N+1) dp[j][k] = ndp[j][k];
	}
	return dp[0][0];
}

int main(){
	init();
	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: 4864