n=int(input())
l=[list(map(int,input().split())) for i in range(n)]
L1=[]
L2=[]
for i in range(n):
  L1.append(l[i][0]+4*l[i][1])
  L2.append(l[i][0]%2)
if len(set(L2))!=1:
  print(-1)
  exit()
m=max(L1)
ans=0
for i in range(n):
  ans+=(m-L1[i])//2
print(ans)