import sys

sys.setrecursionlimit(10 ** 6)
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]

def main():
    md = 10 ** 9 + 7
    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] = 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] = (dp[i + 1][j]+(xx[i] - i + j) * pre)%md
            # 処理しない
            if tt[i]: dp[i + 1][j + 1] = (pre+dp[i + 1][j + 1])%md
    #p2D(dp)
    fac=[1]
    for i in range(1,one+1):fac.append(fac[-1]*i%md)
    ans=0
    for j in range(one+1):
        if (one-j)&1:ans-=dp[n][j]*fac[j]
        else:ans+=dp[n][j]*fac[j]
        ans%=md
    print(ans)

main()