import sys
m = 10**9+7
n = int(input())
l = [list(map(int, s.split())) for s in sys.stdin]
l = sorted((x-t, t) for t, x in l)
c = [1]+[0]*n
d = 1
for i, (x, t) in enumerate(l):
	d += t
	for j in range(d-1, -1, -1):
		c[j] = (c[j-1]*j - c[j]*(x-i+j) if t else c[j]*(x-i+j)) % m
print(sum(c)%m)