n=int(input())
l=[]

l=[list(map(int,input().split())) for _ in range(n)]
*m,=map(lambda x:x[0]+4*x[1],l)

*a,=map(lambda x:x%2,m)
if not (all(a) or not any(a)):
  print(-1)
else:
  b=max(m)
  print(sum(map(lambda x:(b-x)//2,m)))