#!/bin/python3

import sys


n = int(input().strip())
for a0 in range(n):
    x,y = input().strip().split(' ')
    x,y = [int(x),int(y)]
    if y in range(n+1) and x==0:
        #print(x,y)
        continue;
    else:
        #print(x,y)
        print("NO")
        exit(0)
print("YES")