417B - Crash - CodeForces Solution

import sys
from math import *
from collections import Counter
input=sys.stdin.readline

def isSorted(a):
    for i in range(1, len(a)):
        if a[i] < a[i-1]:
            return False
    return True

d={}
n=int(input())
for i in range(n):
    a,b=map(int,input().split())
    a,b=b,a
        if a in d:
        if b in d[a]:
            continue
        elif b-1 in d[a]:
            d[a].add(b)
        else:
            print("NO")
            exit()
    if not a in d:
        if b==0:
            d[a]=set([b])
        else:
            print("NO")
            exit()
print("YES")