from operator import itemgetter
mod = 10 ** 9 + 7
fact = [1] * 3333
for i in range(1, 3333):
    fact[i] = fact[i-1] * i % mod

N = int(input())
down = 0
tX = []
for _ in range(N):
    t, x = map(int, input().split())
    if t == 0:
        down += 1
        tX.append((t, x))
    else:
        tX.append((t, x-1))

tX.sort(key=itemgetter(1))

dp = [0] * (N + 1)
dp[0] = 1
for t, x in tX:
    ndp = [0] * (N + 1)
    for i in range(N):
        ndp[i+1] = dp[i] * max(0, x-i) % mod
    if t == 1:
        for i in range(N+1):
            ndp[i] += dp[i]
            ndp[i] %= mod
    dp = ndp

ans = 0
sgn = 1
for i in range(down, N+1):
    ans += sgn * dp[i] * fact[N-i] % mod
    ans %= mod
    sgn = -sgn
print(ans)