#include<stdio.h>
#include<iostream>
#include<vector>
#include<algorithm>
#include<string>
#include<string.h>

#ifdef LOCAL
#define eprintf(...) fprintf(stderr, __VA_ARGS__)
#else
#define NDEBUG
#define eprintf(...) do {} while (0)
#endif
#include<cassert>

using namespace std;

typedef long long LL;
typedef vector<int> VI;

#define REP(i,n) for(int i=0, i##_len=(n); i<i##_len; ++i)
#define EACH(i,c) for(__typeof((c).begin()) i=(c).begin(),i##_end=(c).end();i!=i##_end;++i)

template<class T> inline void amin(T &x, const T &y) { if (y<x) x=y; }
template<class T> inline void amax(T &x, const T &y) { if (x<y) x=y; }
template<class Iter> void rprintf(const char *fmt, Iter begin, Iter end) {
    for (bool sp=0; begin!=end; ++begin) { if (sp) putchar(' '); else sp = true; printf(fmt, *begin); }
    putchar('\n');
}

template<unsigned MOD_> struct ModInt {
    static const unsigned MOD = MOD_;
    unsigned x;
    void undef() { x = (unsigned)-1; }
    bool isnan() const { return x == (unsigned)-1; }
    inline int geti() const { return (int)x; }
    ModInt() { x = 0; }
    ModInt(const ModInt &y) { x = y.x; }
    ModInt(int y) { if (y<0 || (int)MOD<=y) y %= (int)MOD; if (y<0) y += MOD; x=y; }
    ModInt(unsigned y) { if (MOD<=y) x = y % MOD; else x = y; }
    ModInt(long long y) { if (y<0 || MOD<=y) y %= MOD; if (y<0) y += MOD; x=y; }
    ModInt(unsigned long long y) { if (MOD<=y) x = y % MOD; else x = y; }
    ModInt &operator+=(const ModInt y) { if ((x += y.x) >= MOD) x -= MOD; return *this; }
    ModInt &operator-=(const ModInt y) { if ((x -= y.x) & (1u<<31)) x += MOD; return *this; }
    ModInt &operator*=(const ModInt y) { x = (unsigned long long)x * y.x % MOD; return *this; }
    ModInt &operator/=(const ModInt y) { x = (unsigned long long)x * y.inv().x % MOD; return *this; }
    ModInt operator-() const { return (x ? MOD-x: 0); }

    ModInt inv() const { return pow(MOD-2); }
    ModInt pow(long long y) const {
	ModInt b = *this, r = 1;
	if (y < 0) { b = b.inv(); y = -y; }
	for (; y; y>>=1) {
	    if (y&1) r *= b;
	    b *= b;
	}
	return r;
    }

    ModInt extgcd() const {
	unsigned a = MOD, b = x; int u = 0, v = 1;
	while (b) {
	    int t = a / b;
	    a -= t * b; swap(a, b);
	    u -= t * v; swap(u, v);
	}
	if (u < 0) u += MOD;
	return ModInt(u);
    }

    friend ModInt operator+(ModInt x, const ModInt y) { return x += y; }
    friend ModInt operator-(ModInt x, const ModInt y) { return x -= y; }
    friend ModInt operator*(ModInt x, const ModInt y) { return x *= y; }
    friend ModInt operator/(ModInt x, const ModInt y) { return x *= y.inv(); }
    friend bool operator<(const ModInt x, const ModInt y) { return x.x < y.x; }
    friend bool operator==(const ModInt x, const ModInt y) { return x.x == y.x; }
    friend bool operator!=(const ModInt x, const ModInt y) { return x.x != y.x; }
};

const LL MOD = 1000000007;
typedef ModInt<MOD> Mint;
int N;
int C[611];
int ZERO, ONE;

const int SIZE = 311;
Mint buf[2][SIZE][SIZE];

Mint solve() {
    Mint (*cur)[SIZE] = buf[0];
    Mint (*nxt)[SIZE] = buf[1];
    memset(cur, 0, sizeof buf[0]);

    int zero = 0, one = 0;
    cur[0][0] = 1;
    REP (i, N*2) {
	if (C[i] == 2) continue;

	memset(nxt, 0, sizeof buf[0]);
	if (C[i] == 0) {
	    REP (a, SIZE-1) REP (b, SIZE-1) if (cur[a][b].x) {
		nxt[a+1][b] += cur[a][b];

		int filled = (zero + one - a - b) / 2 + b + ONE - one;
		if (ZERO + ONE > filled) nxt[a][b+1] += cur[a][b] * (ZERO + ONE - filled);

		if (b) nxt[a][b-1] += cur[a][b];
	    } 
	    zero++;
	} else {
	    REP (a, SIZE-1) REP (b, SIZE-1) if (cur[a][b].x) {
		nxt[a][b+1] += cur[a][b];
		if (a) nxt[a-1][b] += cur[a][b] * a;
	    } 
	    one++;
	}
	swap(cur, nxt);
    }
    return cur[0][0];
}

void MAIN() {
    scanf("%d", &N);
    REP (i, N) {
	int x, y;
	scanf("%d%d", &x, &y);
	x--; y--;
	if (x < 0 && y < 0) {
	    ZERO++;
	} else if (x < 0) {
	    ONE++;
	    C[y] = 1;
	} else if (y < 0) {
	    ONE++;
	    C[x] = 1;
	} else {
	    C[x] = C[y] = 2;
	}
    }

    Mint ans = solve();

    printf("%d\n", ans.geti());
}

int main() {
    int TC = 1;
//    scanf("%d", &TC);
    REP (tc, TC) MAIN();
    return 0;
}



Measured memory_bytes: 1024