import sys


N = int(raw_input())
maze = []
visited = []

for i in range(N):
    tmp = []
    visitedTemp = []
    for j in range(N):
        tmp.append(0)
        visitedTemp = []
    maze.append(tmp)
    visited.append(visitedTemp)
i = 0

while i < N:
    inpt = map(int,raw_input().split(' '))
    if len(input) == 1:
        print(-1)
        sys.exit()
    else:
        Y = inpt[1]-1
        X = inpt[0]-1
        maze[Y][X] =1
    i +=1