n = int(input())
X = [[] for i in range(2)]
for _ in range(n):
    t,x = map(int,input().split())
    X[t].append(x-1)

mod = 10**9+7
A = sorted(X[0])
B = sorted(X[1])

la = len(A)
lb = len(B)
dp = [[0]*(n+1) for i in range(n+1)]
dp[0][0] = 1

for i in range(n):
    c0 = 0
    c1 = 0
    for x in A:
        if i <= x:
            c0 += 1
    for x in B:
        if i >= x:
            c1 += 1
    
    for x in range(la+1):
        y = i-x
        if y > lb or y < 0:
            continue
        
        if x < la:
            dp[x+1][y] += dp[x][y]*(c0-(la-x-1))%mod
            dp[x+1][y] %= mod
        if y < lb:
            dp[x][y+1] += dp[x][y]*(c1-y)%mod
            dp[x][y+1] %= mod

ans = 0
for i in range(n+1):
    ans += dp[i][n-i]
    ans %= mod
print(ans)