n=int(input())
ans=[];t=0;chk=0
s=[]
for _ in range(n):
    a,b=map(int,input().split())
    ans.append(a+b*4)
    s.append((a+b*4)%2==0)
chk=max(ans)-min(ans)
if chk==0:
    print(0)
elif sum(s)==n or sum([i==0 for i in s])==n:
    for i in range(n):
        t+=(max(ans)-ans[i])//2
    print(t)
else:
    print(-1)