import sys
input = lambda : sys.stdin.readline().rstrip()

sys.setrecursionlimit(2*10**5+10)
write = lambda x: sys.stdout.write(x+"\n")
debug = lambda x: sys.stderr.write(x+"\n")
writef = lambda x: print("{:.12f}".format(x))


n = int(input())
l = [0]*n
r = [0]*n
ll = 0
for i in range(n):
    t,x = list(map(int, input().split()))
    x -= 1
    if t==1:
        r[x] += 1
    else:
        l[x] += 1
        ll += 1
dp = [0]*(n+1)
dp[0] = 1
M = 10**9+7
vl = vr = 0
for i in range(n):
    for _ in range(r[i]):
        ndp = dp[:]
        for j in range(n):
            num = i-(vl+j)
            if num<=0:
                continue
            ndp[j+1] += dp[j]*num%M
            ndp[j+1] %= M
        dp = ndp
        vr += 1
    for _ in range(l[i]):
        ndp = [0]*(n+1)
        for j in range(n):
            num = i+1-(vl+j)
            if num<=0:
                continue
            ndp[j] += dp[j]*num%M
            ndp[j] %= M
        vl += 1
        dp = ndp
g1 = [1]
for i in range(n+10):
    g1.append(g1[-1]*(i+1)%M)
ans = 0
s = -1
for j in range(n+1):
    s *= -1
    if dp[j]==0:
        continue
    ans += s * (dp[j] * g1[vr-j] % M)
    ans %= M
print(ans%M)