import sys

int1 = lambda x: int(x) - 1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.readline())
def MI(): return map(int, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def SI(): return sys.stdin.readline()[:-1]

class mint:
    def __init__(self, x):
        self.__x = x % md

    def __str__(self):
        return str(self.__x)

    def __add__(self, other):
        if isinstance(other, mint): other = other.__x
        return mint(self.__x + other)

    def __sub__(self, other):
        if isinstance(other, mint): other = other.__x
        return mint(self.__x - other)

    def __rsub__(self, other):
        return mint(other - self.__x)

    def __mul__(self, other):
        if isinstance(other, mint): other = other.__x
        return mint(self.__x * other)

    __radd__ = __add__
    __rmul__ = __mul__

    def __truediv__(self, other):
        if isinstance(other, mint): other = other.__x
        return mint(self.__x * pow(other, md - 2, md))

    def __rtruediv__(self, other):
        return mint(other * pow(self.__x, md - 2, md))

    def __pow__(self, power, modulo=None):
        return mint(pow(self.__x, power, md))

md = 10**9+7

def main():
    n = II()
    xt = []
    one = 0
    for _ in range(n):
        t, x = MI()
        if t:
            x -= 1
            one += 1
        xt.append((x, t))
    tt = []
    xx = []
    for x, t in sorted(xt):
        tt.append(t)
        xx.append(x)
    #print(tt)
    #print(xx)
    # dp[i][j]...i個まで見たとき、t=1の処理をj個飛ばしたときの場合の数
    dp = [[0] * (one + 1) for _ in range(n + 1)]
    dp[0][0] = mint(1)
    for i in range(n):
        for j in range(min(i, one) + 1):
            pre = dp[i][j]
            if pre == 0: continue
            # 処理する
            dp[i + 1][j] += (xx[i] - i + j) * pre
            # 処理しない
            if tt[i]: dp[i + 1][j + 1] += pre
    #p2D(dp)
    fac=[mint(1)]
    for i in range(1,one+1):fac.append(fac[-1]*i)
    ans=mint(0)
    for j in range(one+1):
        if (one-j)&1:ans-=dp[n][j]*fac[j]
        else:ans+=dp[n][j]*fac[j]
    print(ans)

main()