N=int(input())
mod=10**9+7

T0=[]
T1=[]
for i in range(N):
    t,x=map(int,input().split())
    if t==0:
        T0.append(x)
    else:
        T1.append(x)

T0.sort()
T1.sort()

UP0=[0]*(N+1)
DOWN1=[0]*(N+1)

for t in T0:
    UP0[t]+=1

for i in range(N-1,-1,-1):
    UP0[i]+=UP0[i+1]

for t in T1:
    DOWN1[t]+=1

for i in range(1,N+1):
    DOWN1[i]+=DOWN1[i-1]

DP=[[0]*(N+1) for i in range(N+1)]
DP[0][0]=1

for SUM in range(1,N+1):
    for t_0 in range(min(SUM+1,len(T0)+1)):
        t_1=SUM-t_0

        if t_0>=1 and UP0[SUM]-(len(T0)-t_0)>=0:
            DP[t_0][t_1]+=DP[t_0-1][t_1]*(UP0[SUM]-(len(T0)-t_0))
        if t_1>=1 and DOWN1[SUM]-(t_1-1)>=0:
            DP[t_0][t_1]+=DP[t_0][t_1-1]*(DOWN1[SUM]-(t_1-1))
        DP[t_0][t_1]%=mod

print(DP[len(T0)][len(T1)]%mod)