import sys
input=lambda: sys.stdin.readline().rstrip()
n=int(input())
A=[]
for _ in range(n):
  a,b=map(int,input().split())
  A.append(a+4*b)
A.sort()
a=A[-1]
ans=0
chk=True
for i in range(n-1):
  if (a-A[i])%2:
    chk=False
    break
  else:
    ans+=(a-A[i])//2
if not chk:
  print(-1)
else:
  print(ans)